TSUNG-HSUN T. HSIANG1 SIX FIGURES) · Thatthe orchid pollinium is a rich source of growth substances and there-fore capable of inducingparthenocarpy andprevention of leaf abscission

PHYSIOLOGICAL AND BIOCHEMICAL CHANGES ACCOM-PANYING POLLINATION IN ORCHID FLOWERS.

I. GENERAL OBSERVATIONS ANDWATER RELATIONS

TSUNG-HSUN T. HSIANG1

(WITH SIX FIGURES)

Received September 11, 1950

Introduction

It is well known that the reproductive processes in plants are associatedwith pronounced changes of the physiology of the whole plant. In a seriesof papers, MURNEEK (19), MURNEEK and WITTWER (20, 21), WITTWER andMURNEEK (28) discussed mechanisms whereby synapsis and syngamy couldextend their effects throughout the whole plant body. The increased knowl-edge about parthenocarpy has called attention to physiological distinctionsbetween the processes of pollination and fertilization. The effect of pollina-tion in leading to fruit development has been duplicated by use of syntheticgrowth substances (10). Fertilization is essential for embryo initiation andso far has not been replaced by treatment with any known substances. It istherefore of interest to know what effect pollination alone has on themetabolism of the flower.

In most plants under normal conditions, pollination is followed bygametic union. The fact that the two events closely follow one anothermakes it difficult to separate them for experimental purposes. The orchidsare exceptions to the general rule; the extremely great time lag betweenmicrogametophyte and megagametophyte development in these plants hasmade them favorable material for "pollination effect" studies. In mostorchids, pollination is necessary to start the development of the megaga-metophyte which is wholly undeveloped at the time of anthesis. The twoprocesses are usually separated by a period of several months. The struc-ture of the orchid flower offers an additional advantage for physiologicalinvestigations. The large size of the perianth and the column, a unitedstructure composed of the stamens and the pistil, makes it suitable forstudies of metabolism.

Although the "post-pollination phenomenon" has been known sinceFitting's early work (8) on orchids, it has received relatively little furtherattention. The purpose of the present investigation is to find out the im-mediate effects of pollination, with a view to seeing to what extent theseeffects can be explained in terms of auxin activity. It is hoped to integratethe physiology of pollination and to derive the mechanism of its action fromthe results obtained.

1 Present address: Department of Biology, Morehouse College, Atlanta, Ga.441

Copyright (c) 2020 American Society of Plant Biologists. All rights reserved.

PLANT PHYSIOLOGY

LiteratureFITTING (8, 9) demonstrated that the placement of pollen, living or dead,

on the stigma of orchids caused premature wilting of the perianth, closing ofthe stigma, swelling of the column and sometimes the swelling of the ovary.MORITA (18) studied the effects of application of dead pollen, foreign pollenand pollen extracts in Cymbidium. LAIBACH (13) reported that in Calanthe,in addition to the post-pollination phenomena shown by Fitting, dead orliving pollen placed on the stigma produced curvature changes of the pedicel.That the orchid pollinium is a rich source of growth substances and there-fore capable of inducing parthenocarpy and prevention of leaf abscission hasbeen demonstrated by LAIBACH (14, 15), LAIBACH and MASCHMANN (16),and MAI (17). HUBERT and MATON (11) studied the relative effectivenessof a wide variety of chemical substances in inducing the post-floral phe-nomena in tropical orchids. SESHAGIRIAH (23) observed an abundance ofstarch in pollinated ovaries of Habenaria and concluded that growth sub-stances from the pollen stimulated starch synthesis. CURTIS (3) reportedthat, in some species of Phalaenopsis, the petals do not wilt after pollina-tion, but rather become thickened and turn green.

WHITE (25) studied the respiratory activity of pollinated and non-polli-nated flowers of a wide variety of plants not including orchids. In almostevery plant she examined, pollination not only produced a rapid rise inrespiratory activity, but also markedly raised the respiratory quotient.DETURK et al. (4) noted that the beginning of phytin synthesis followedimmediately after pollination, from which they concluded that pollen acti-vated the zymogen of the enzyme responsible for phytin synthesis. Thelater work of EARLEY and DETURK (5) added some corroborative evidencesupporting the above statement.

Materials and methods

In the course of this experiment more than a thousand flowers of culti-vated orchids were studied. Potted plants, cut inflorescences and cutflowers, were provided through the exceptional courtesy of Dr. C. K. Schu-bert from his private orchid collection in Madison, Wisconsin. Some plantswere raised in the greenhouses of the Botany Department of the Universityof Wisconsin.

Normally the flowers of a single inflorescence open at about the sametime. They have approximately equal weights and are uniform in appear-ance. These facts made it possible to study the effects of various treat-ments on presumably comparable flowers from the same inflorescence.

When sufficient flowers of uniform origin were not available for an entireexperiment, particular care was taken to choose like flowers for each sub-experiment, that is, each sampling, to minimize the variation not caused bytreatment. In sueh cases, separate controls were always used for differentsub-experiments.

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HSIANG: POLLINATION IN ORCHID FLOWERS

Throughout the work, the normal complement of pollinia that is presentin a particular species was used to pollinate one flower, e.g., four in Cattleyaand Dendrobium, eight in Laeliocattleya, etc. The pollinia usually camefrom flowers other than the one being pollinated, although the origin of thepollinia did not seem to make much difference.

Two growth substances were used in treating stigmas: indole-3-aceticacid and p-naphthalene acetic acid, both at a concentration of 0.5% in lano-lin paste. About 5 mm.3 of paste was placed on the stigma of each flowertreated.

The following symbols are used throughout the descriptions: Cont., Pol.,NAA and IAA stand respectively for control, pollinated, naphthalene aceticacid treatment and indole acetic treatment, the letters C and P for the floralparts column and perianth. Thus, Cont-P means control perianth, NAA-Cnaphthalene acetic acid treated column, etc.

Almost all of the data presented are representative of many replicatesets. Unless otherwise specified, each determination involves the use of twoor three large flowers such as Cymbidium or Cattleya labiata, and four orfive small flowers such as Dendrobium or Cattleya Bowringiana.

Results

EXTERNAL CHANGES AND THEIR QUANTITATIVE ANALYSES

In all the species studied, pollination brought about wilting of the peri-anth, local swelling of the column resulting in imprisonment of the pollinia,followed by enlargement of the whole column. In Cymbidium, as an im-mediate effect of pollination, the lip of the perianth as well as the inner orventral side of the column turned from white to red. The color changeoccurred approximately 48 hours after the application of pollen and itsintensity increased acropetally. The red pigment was proved to be antho-cyanin. The appearance of color was not associated with any change insap pH. All these visible effects of pollination were qualitatively duplicatedby application of 0.5% NAA and IAA.

To approach the quantitative aspects of the swelling and wilting, a studywas made of the relative changes in fresh and dry weights of the columnand perianth. The reason for studying them separately is obvious. Withpollination the life of the perianth ends; pollination sets off further growthof the column.

After treatments flowers were cut at the base of the ovary. As theovary of most species is longitudinally grooved, the lower end of the grooveswas taken as. the demarkation between the ovary and the pedicel. Thefloral parts were weighed separately, dried at 80 to 100', C for about 24hours and reweighed.

Pollination brought about increases in both the fresh and dry weightsof the column. Since in earlier experiments a continuous growth of thecontrol column after anthesis was indicated, later data were expressed in

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per cent. of the control, so that the changes within the controls could beeliminated. Figure 1 shows the gain in fresh and dry weights of Cymbidiumcolumns following pollination and NAA treatment. It is seen that, on apercentage basis, the increase of fresh weight was greater than that of dryweight at the beginning. Later on this relationship was reversed. Theseobservations indicate an initial dominance of water uptake over the increasein dry matter. The same general pattern is shown in more than 20 experi-ments with different species. When the effect of NAA was compared withthat of pollen (fig. 1), the NAA effect considerably exceeded that of pollenat the beginning, especially in increasing the fresh weight. The leveling offof the curve which soon occurred indicated that the pollen was becomingrelatively more effective. The same relationship is shown in figure 2 whichrecords the relative effectiveness of NAA, IAA, and pollen in bringing about

POL.-Ca: -/NAA-C

50 100HOURS AFTER TREATMENT

FIG. 2. Relative effectiveness of pollen, IAA, and NAA in producing dry weightincrease in the column. Cattleya labiata.

the dry weight increase for Cattleya labiata. NAA had a greater effectthan IAA.

The perianth not only loses water, but also dry matter. It is note-worthy that, while the fresh weight of the perianth underwent almost alinear decrease with time, the dry weight seems to be much more steady.

EXPERIMENTS ON PICKED FLOWERS

Flowers of Dendrobium nobile were cut from the plant and their pedicelsimmersed in distilled water. A wire screen fitted on a glass container wasused to support the flowers. One set of these flowers was then pollinated.Figure 3 includes photographs taken 115 hours after treatments. The closingof the perianth of the flowers in the pollinated group is shown. Owing tothe drying out of the tissue, the veins on the petals of the pollinated flowersare clearly visible.

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PLANT PHYSIOLOGY

The same relationship exists here between the percentage change in freshand dry weight as revealed previously by flowers still attached to the plant,except that the magnitude of both the increase and decrease was greatlylimited.

Following these preliminary observations of the apparent changes, effortswere made to determine: (1) the mechanisms governing the increased wateruptake of the column and the loss of water from the perianth; (2) the rela-tive shift of dry matter between the perianth and the column, their quanti-tative relationships and the components that are directly concerned; and(3) changes in the more fundamental metabolic processes such as respira-~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~. ... ......t:is

FIG. 3. Picked flowers of Dendrobium nobile. Left: 115 hours after pollination.Right: Non-treated, unpollinated, otherwise same as those shown at left.

tion and enzymatic activity. This paper reports the findings on waterrelations. In a paper that will appear later the changes in respiration,catalase activity, and chemical constituents will be described.

MEASUREMENT OF OSMOTIC PRESSUTRE

Osmotic pressure was determined cryoscopically. Treated floral partswere cut from the plants and frozen with solid carbon dioxide. After thaw-ing, the tissues were cut into small pieces and ground in a mortar beforebeing subjected to pressing in a hydraulic press at a pressure of 8000 lb./sq.inch. Care was taken to prevent loss of water due to evaporation whichwould particularly affect the results with small samples. It took three

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flowers of Cattleya labiata and eight of C. Bowringiana to get sufficient sapfor one determination. A mixture of chipped ice and CaCl2 was found satis-factory for use in the freezing bath. Freezing point determinations weremade with a Drucker-Burian microthermometer.

The results are summarized in table I. A slight increase of osmoticpressure in the column was shown 115 hours after pollination and auxintreatment. The change in osmotic pressure of the perianth followed thesame pattern.

UPTAKE OF DISTILLED WATER BY CUT DISCS

Experiments were carried out to compare the ability of detached flowersto take up water. For this purpose, the flowers were severed from theplants at various intervals after treatment and separated into column andperianth. The respective parts were hand-sectioned, the weights of thesections obtained and the sections put into bottles containing enough dis-

TABLE IEFFECTS OF POLLINATION AND AUXIN TREATMENT ON OSMOTIC PRESSURE

OF FLORAL PARTS. DETERMINATIONS WERE MADE115 HOURS AFTER TREATMENT.

Osmotic pressure in atmospheresPart and Cattleya labiata C. Bowringianatreatment

Expt. 23 Expt. 25 Expt. 23 Expt. 25

Cont-C 5.99 5.76 5.91 4.43Pol-C 6.80 6.34 6.37 ....

NAA-C .... .... ... 5.16

Cont-P 5.22 5.93 4.33 5.40Pol-P 6.49 .... 5.37NAA-P .... .... .... 6.29IAA-P 6.82 ....

tilled water to cover them. The sections turned brown very quickly incontact with air, the pollinated ones at a greater rate and intensity. Noattempts were made to prevent oxidation, since any such effort might resultin an interference with water uptake. The sections were taken from thewater after two to three hours and reweighed after the adhering wvater hadbeen blotted off with filter paper. The gains in weight are recorded as apercentage of the original weight in the left-hand portion of table II. It isobvious that the column of the pollinated flower took up significantly morewater from the medium.

It was suspected that equilibrium might not have been reached at thetime determinations were made, and that the observed differences in theamount of water uptake might disappear when more time was allowed.However, a study of the rate of water uptake with Laeliocattleya showedthat the treated and non-treated tissues reached their saturation point atabout the same time. Figures 4 and 5 show that the percentage difference

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was established significantly within 30 minutes after the tissues wereimmersed in water.

To determine whether this accelerated water uptake was in any wayconnected with aerobic processes, experiments were run in an atmosphere ofnitrogen. Commercial nitrogen was purified by passing through the follow-ing solutions: sodium hydrosulphite (with indicator, sodium anthraquinone,B sulphonate, added) to absorb oxygen; NaOH to absorb carbon dioxide.This system was finally connected to a water trap to regulate the rate of gas

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FIG. 5. Rate of water uptake of detached columns from differently treated flowers.Each point on each curve represents increase in weight in percentage of initial weight.Laeliocattleya.

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PLANT PHYSIOLOGY

flow. In the earlier experiments, nitrogen was passed to a closed jar. Sec-tions from differently treated flowers were put into small bottles. Only asmall amount of water was added so as to hold at a minimum the amountof dissolved oxygen. The bottles were put into the jar which had beenflushed with a nitrogen stream for a sufficient period of time. In laterexperiments, the nitrogen gas was passed continuously through the bottlesat a moderate rate during the experimental periods of two to three hours.The percentage increase in weight of the tissues is recorded in the right-handportion of table II.

It is clear that in an atmosphere of nitrogen the columns from pollinatedand non-pollinated flowers took up the same amount of water on a percent-age basis. In other words, the effects of pollen and NAA on water uptakedisappear in the absence of oxygen.

POSSIBLE ROLE OF IMBIBITION

Ovaries of pollinated flowers of many species of orchids seem to containmore mucilaginous substances than those of non-pollinated flowers. Thisfact was especially noticeable during the process of sectioning and grindingthe experimental material. This observation led to the speculation thatimbibition by such substances might explain part of the increase in wateruptake.

To investigate this possibility, water uptake by killed tissues was studied.Fresh columns were weighed and killed by steaming, after which they weresoaked in distilled water. As can be seen in table III, at the end of threehours, both the pollinated and the non-pollinated tissues lost some weightto the medium, but the latter to a much lese extent than the former.

TABLE IIIWATER LOSS FROM COLUMNS OF Dendrobium nobile FROM POLLINATED AND

NON-POLLINATED FLOWERS. KILLED BY STEAMINGONE WEEK AFTER POLLINATION.

Tissue Original fresh Weight after 3 hrs. % decreaseweight, gms. in water, gms. in weight

Cont-C 0.5285 0.3877 26.64Pol-C 0.7575 0.6466 14.64

While this is not conclusive proof of the role played by imbibition in theaccelerated water uptake, it indicates that after pollination the column hasa greater water-holding capacity.

Loss OF WATER FROM PERIANTH-TRANSPIRATION

The transpiration of the flowers was studied in the following manner.

Flowers of like size and age were cut from the plant, treated and weighed.The pedicels of the flowers were immediately inserted into the rubber capsof small flasks or vials filled with distilled water. The holes in the rubbercaps were made to fit the pedicels. To ensure complete sealing, the flower-

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to-rubber and rubber-to-glass connections were sealed with a mixture ofparaffin and beeswax. The flasks were weighed at intervals of about 12hours. The decrease in weight is taken as the loss of water through theflower, presumably mainly the perianth. Figure 6 plots the loss of waterper unit time per unit weight of the flower against time. The choice of unitweight here is thought to be reasonable because the petals are flat and canthus be taken as a 2-dimensional structure and any change in surface wouldbe reflected in the weight.

It is readily seen that both pollination and auxin treatment evenly andcontinuously stimulated transpiration from the beginning to the end of theexperiment. Eight different experiments showed the same general result.

Microscopic examinations revealed no stomata in the epidermis of theperianth. Therefore it must have been the epidermal or cuticular transpi-ration which was affected.

Discussion

Evidence has been presented to show that the placement of pollen on thestigma greatly shifts the water balance of the flower. The column swellsmainly due to increased capacity to absorb water; the perianth wilts mainlybecause of increased transpiration.

The application of NAA and IAA in place of pollen to the stigma pro-duces qualitatively the same results. We shall discuss the mechanism ofpollination, together with the comparison of the pollen effects and auxineffects, in the second paper, and confine our discussion here to the effects ofpollen or auxin on water uptake and transpiration.

The effect of auxin on water uptake has been summarized in reviewpapers by VAN OVERBEEK (27) and SKOOG (24). In the present work, auxindid not cause a decrease of osmotic pressure as reported with potato tubersby VAN OVERBEEK (26). However, the same conclusion is reached that theslightly increased osmotic pressure is far from being enough to explain theenhanced water uptake. This increased water uptake is made evident inthe apparent swelling of the column and definitely proved by the experi-ments on cut discs of column tissue in water.

It is shown that cut discs from columns of treated flowers absorb morewater than those from columns of non-treated flowers in air, this differencedisappears in a nitrogen atmosphere. This dependence of pollen- and auxin-stimulated water uptake on oxygen is in agreement with the results obtainedby KELLY (12) in Avena coleoptile segments.

As to the effect of auxin on transpiration of plants, there seems to be alack of agreement among the earlier authors. AMLONG (1) reported anincreased transpiration in tomato plants from seeds that had been soakedin a solution containing potassium naphthalene acetate. BROWN (2) pre-sented data showing the amount of water transpired by normal bean seed-lings and by those sprayed with 2,4-D and concluded that the treated seed-lings transpired significantly less water than the non-treated ones. Theresults of FERRI and LEX (6), FERRI and RACHID (7) are also in favor of

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the conclusion that treatment of plants with growth substances generallyreduces water loss from the plants. Lately, PLAYER (22) noticed a decreaseof total transpiration in castor bean plants sprayed with various syntheticgrowth substances, although the total transpiration of .com plants was notaffected by these sprays.

In the present investigation, the method of application of auxin is differ-ent from all the earlier ones, and the plant parts concerned are also different.Auxin is applied to the stigma and the transpiration of the perianth isstudied. This may not be comparable to spraying the plants and studyingthe transpiration of the leaves. However, our conclusion that auxin treat-ment increases transpiration rate would not conflict with BROWN'S (2) con-clusion if he had taken leaf surface into consideration. He shows in histable 1 that the loss of water per 10 plants in five days is 75.41 and 49.88grams respectively for the untreated and treated seedlings. But during thisperiod, the untreated plants have grown to be twice as large with a leafsurface twice as great (see his table 4). Recalculation of his data with con-sideration of the leaf surface gives a relative transpiration rate of roughly37.5 to 50 for the untreated and the treated. An acceleration of transpira-tion rate is therefore caused by the treatment with 2,4-D.

SummaryPollination was shown to bring about the external visible changes reported

by earlier workers as to the wilting of perianth, the closing of the stigma,the enlargement of the column, and sometimes the swelling of the ovary.In Cymbidium, as an immediate effect of pollination, the lip of the perianthas well as the inner side of the column turned from white to red. The redpigment was proved to be anthocyanin. The appearance of color was notassociated with any change in sap pH.

Pollination brought about increases in both fresh and dry weights of thecolumn. On a percentage basis, the increase in fresh weight was greaterthan that of dry weight at the beginning; later on this relationship wasreversed. The greater per cent. increase of fresh weight over dry weight,followed by a reversion of this relationship, indicates an initial dominanceof water uptake over the increase in dry matter.

The perianth after pollination not only lost water, but also dry matter.Cut flowers responded to pollination in essentially the same manner.

Osmotic pressure measured 115 hours after pollination showed a slightincrease over the control. This slight observed difference in osmotic pres-sure did not appear to be sufficient to account for the difference in theamount of water uptake.

In air, cut discs of columns from pollinated flowers took up more wateron a percentage basis than those discs of columns from non-pollinatedflowers. This difference was significantly established within 30 minutesafter the tissues were immersed in water. However, in a nitrogen atmos-phere, this difference disappeared. It is concluded that the pollen- or auxin-stimulation of water uptake is related to aerobic processes.

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Columns from pollinated flowers were shown to have a greater water-holding capacity. It is postulated that pollination causes an increase ofhydrophilic colloids in the column tissue.

The wilting of the perianth after pollination and auxin treatment resultsfrom an increased rate of epidermal transpiration.

The author wishes to express her gratitude and appreciation to Dr. C. K.Schubert of Madison, Wisconsin, without whose exceptional kindness incontributing the flowers from his private orchid collection the investigationwould have been impossible; to Dr. John T. Curtis for suggesting the prob-lem and for his interest and helpful guidance throughout the course of thestudy; to Dr. J. F. Stauffer for his help in various ways; to Dr. Hilda F.Rosene for her constructive criticism on the part of water relations; to Dr.W. Gordon Whaley for reading the manuscript. She is also indebted for thefellowships awarded during her studies at the University of Wisconsin.

DEPARTMENT OF BOTANYUNIVERSITY OF WISCONSIN

MADISON, WISCONSIN

LITERATURE CITED1. AMLONG, H. U. tber den Einfluss der Hormonisierung auf die Tran-

spiration der Pflanze. Naturwiss. 31: 44-45. 1943.2. BROWN, J. W. Effect of 2,4-dichlorophenoxyacetic acid on water rela-

tions, the accumulation and distribution of solid matter, and therespiration of bean plants. Bot. Gaz. 107: 332-343. 1946.

3. CURTIS, J. T. An unusual pollen reaction in Phalaenopsis. Amer.Orchid Soc. Bull. 11: 593617. 1940.

4. DETURK, E. E., HOLBERT, J. R., and HAUK, B. W. Chemical transfor-mation of phosphorus in the growing corn plant with results on twofirst-generation crosses. Jour. Agr. Res. 46: 121-141. 1933.

5. EARLEY, E. B. and DETURK, E. E. Time and rate of synthesis ofphytin in corn grain during the reproductive period. Amer. Soc.Agr. Jour. 36: 803-814. 1944.

6. FERRI, M. G. and LEX, A. Stomatal behavior as influenced by treat-ment with ,B naphthoxyacetic acid. Contr. Boyce Thompson Inst.15: 283-290. 1948.

7. FERRI, M. G. and RACHID, M. Further information on the stomatalbehavior as influenced by treatment with hormone-like substances.Anais. Acad. Bras. Cienc. 21: 155-166. 1949.

8. FITTING, H. Die Beeinflussung der Orchideenbliuten durch die Bestiiu-bung durch audere Umstande. Zeits. Bot. 1: 1-86. 1909.

9. FITTING, H. Entwickelungs physiologische Untersuchungen an Orchi-deenbliuten. Zeits. Bot. 2: 225-266. 1910.

10. GUSTAFSON, F. G. Parthenocarpy: natural and artificial. Bot. Rev.8: 599-654. 1942.

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11. HUBERT, B. and MATON J. The influence of synthetic growth-con-trolling substances and other chemicals on the post-floral phenome-non in tropical orchids. Biologisch Jaarboek 6: 244-285. 1939.

12. KELLY, S. The relationship between respiration and water uptake inthe oat coleoptile. Amer. Jour. Bot. 34: 521-526. 1948.

13. LAIBACH, F. Untersuchungen iuber die Postfloration tropischer Orchi-deen. Planta 9: 341. 1929.

14. LAIBACH, F. Pollenhormone and Wuchsstoff. Der. deut. bot. Ges. 50:383-390. 1932.

15. LAIBACH, F. Wuchsstoffversuche mit Wuchsstoffpaste. Ber. deut. bot.Ges. 51: 33. 1933.

16. LAIBACH, F. and MASCHMANN, E. tJber den Wuchsstoff der Orchideenpollinien. Jahrb. Wiss. Bot. 78: 309-340. 1933.

17. MAI, G. Koorelation untersuchungen an entspreitelen Blattstielenmittels lebenden Orchideenpollinien als Wuchsstoffquelle. Jahrb.Wiss. Bot. 79: 681-713. 1934.

18. MORITA, K. Influence de la pollinisation et d'autres actions exterieuressur la fleur du Cymbidium virens Lindl. Bot. Mag. Tokyo 32: 39.1918.

19. MURNEEK, A. E. Recent advances in physiology of reproduction. Sci-ence 86: 4347. 1937.

20. MURNEEK, A. E. and WITTWER, S. H. Relation of sexual reproductionto development of horticultural plants. I. General effects of flowerand fruit production. Proc. Amer. Hort. Sci. 40: 201-204. 1942.

21. MURNEEK, A. E. and WITTWER, S. H. Synapsis and syngamy as stimu-lating processes of plant development. Science 98: 384-385. 1943.

22. PLAYER, M. A. Effects of some growth regulating substances on thetranspiration of Zea Mays L. and Riccinus communis L. PlantPhysiol. 25: 469-477. 1950.

23. SESHAGIRIAH, K. N. Physiology of pollination in Orchidaceae. Cur-rent Science 10: 30-32. 1941.

24. SKoOG, F. Growth substances in higher plants. Ann. Rev. Biochem.16: 529-564. 1947.

25. WHITE, J. The influence of pollination on the respiratory activity ofthe gynaecium. Ann. Bot. 21: 487-499. 1907.

26. VAN OVERBEEK, J. Auxin, water uptake and osmotic pressure in potatotissue. Amer. Jour. Bot. 31: 265-269. 1944.

27. VAN OVERBEEK, J. Growth regulating substances in plants. Ann. Rev.Biochem. 13: 631466. 1944.

28. WITTWER, S. H. and MURNEEK, A. E. Relation of sexual reproductionto development of horticultural plants. II. Physiological influenceof fertilization (gametic union). Proc. Amer. Soc. Hort. Sci. 40:205-208. 1942.

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Documents

TSUNG-HSUN T. HSIANG1 SIX FIGURES) · Thatthe orchid pollinium is a rich source of growth substances and there-fore capable of inducingparthenocarpy andprevention of leaf abscission