In recent experiments involving the germination of the pollen grains of Vinca rosea, it was observed that relatively poor germination occurred in isolated grains and among those grains aggregated in small groups (Beams and King, 1944). These preliminary observations seemed to warrant a further study of this problem. Accordingly, experiments were designed to determine some of the effects of population density on the percentage germination of these cells

Beams, H. W.

King, R. L.

English

Name not available

Pollen Germination in Relation to Group Size

University of Northern Iowa

Proceedings of the Iowa Academy of ScienceVolume 54 | Annual Issue Article 131947Pollen Germination in Relation to Group SizeH. W. BeamsState University of IowaR. L. KingState University of IowaCopyright © Copyright 1947 by the Iowa Academy of Science, Inc.Follow this and additional works at: https://scholarworks.uni.edu/piasThis Research is brought to you for free and open access by UNI ScholarWorks. It has been accepted for inclusion in Proceedings of the Iowa Academyof Science by an authorized editor of UNI ScholarWorks. For more information, please contact scholarworks@uni.edu.Recommended CitationBeams, H. W. and King, R. L. (1947) "Pollen Germination in Relation to Group Size," Proceedings of the Iowa Academy of Science: Vol.54: No. 1 , Article 13.Available at: https://scholarworks.uni.edu/pias/vol54/iss1/13Pollen Germination in Relation to Group Size H. W. BEAMS AND R. L. KING In recent experiments involving the germination of the pollen grains of Vin<'a rosea, it was observed that relatively poor germina-tion occurred in isolated grains and among those grains aggregated in small groups (Beams and King, 1944). These preliminary obser-vations seemed to warrant a further study of this problem. Accord-ingly, experiments were designed to determine some of the effects of population density on the percentage germination of these cells. Pollen grains from a single anther of Vinca rosea grown in the laboratory during the months of March or April were used in each set of experiments. When taken from the anther, the pollen grains are held together by a yellowish sticky substance. The grains were .Placed on a medium composed of 2% agar and 10% sucrose in petri dishes. Upon being placed on the culture medium, the sticky sub-stance dissolves freeing the grains which immediately imbibe water. Microscopic examination of the grains at this stage discloses three triangularly placed pores in the exine from only one of which usually a tube emerges at germination. After standing from 2 to 4 hours in covered petri dishes at 25°C. in the dark, the grains were exam-ined for the presence of pollen tubes, the criterion used here to de-termine germination. \Vhen the pollen grains are sown on an agar plate, some fall singly, some are in groups of two, three, etc. Germination was tabulated according to the number of pollen grains in a group. The combined results of three such experiments are listed in Table I. It will be observed that only 1.3% of the singly isolated grains germinated and that there is a gradually increasing percentage of germination, as the group becomes larger, up to about eight or ten grains in the group. Here the germination becomes constant at about 70%. Our studies do not permit us to enter far into a discussion of the probable explanation of these results. However, it seems reasonable to assume that some readily diffusible growth promoting substance or substances are given off by the pollen grain. When only a few grains are in a group, the concentration of these is below the thresh-old of germination. If this be true, a greater concentration of the diffusible substance responsible for germination would be present when the grains are in larger groups; which is responsible for a higher percentage of germination in such larger groups. This view is supported by the work of Beck and Joly ( 1941), that better germination and longer pollen tubes occurred in thickly populated cultures ( 40 to 100 grains) than in sparsely populated ones ( 4 to 8 grains). They believe that the relationship between the number of grains seeded and the power of germination is directly associated with the concentration of auxin present. The auxin is 127 1Beams and King: Pollen Germination in Relation to Group SizePublished by UNI ScholarWorks, 1947128 IOWA ACADEMY OF SCIENCE [VOL. 54 thought to be associated with the sticky oily material surrounding the grains. However, Brink ( 1924), was unable to substantiate this view. He concludes that the substance promoting increased growth in pollen tubes is apparently a product of the metabolism of the tubes themselves which is utilized more completely when the tubes are massed. The addition of certain plant materials to the culture medium such as crushed stigma, yeast or raw potato has been found to in-crease tube growth, but the mechanism of this effect is not clearly understood (Brink, 1924). Haberlandt, according to Harrison ( 1928), has observed that in-crease in sizP. occurs in isolated plant cells but division does not occur. In addition, he reports a direct relationship between the size of the piece of plant tissue transplanted, or the number of cells within it, and the number of cell divisions which take place. He concludes from these experiments that the factor responsible for inciting cell division is in the nature of a "wound hormone." Harri-son, too, states that "It is a very interesting and at present inex-plicable fact that single somatic cells isolated in culture media do not poliferate." In many other animal cells, also, it has been observed that the population density may have a marked effect upon growth (Allee, 1931). For example, it has been demonstrated that the eggs of Arbacia cleave more slowly when placed singly than they do when placed in small groups (Allee and Evans, 1937). A similar condi-tion has been recently described for the frog egg (Merwin, 1945). This acceleration takes place even though every effort is made to control such environmental conditions as temperature, carbon dioxide and oxygen concentrations. The percentage of fertility of scattered frog eggs is also lower than that of the bunched ones. In this connec-tion Merwin has ably stated that the factor or factors causing ac-celeration of cleavage in small bunches of frog eggs over that of an isolated egg is unknown. Finally, it may be said that such group-initiated modifications in germination, growth, etc., undoubtedly exist, but that no single ex-planation is probably valid for the phenomena in different groups of animals and plants. ZOOLOGY DEPARTMENT, STATE UNIVERSITY OF IOWA. 2Proceedings of the Iowa Academy of Science, Vol. 54 [1947], No. 1, Art. 13https://scholarworks.uni.edu/pias/vol54/iss1/131947] POLLEN GERMINATION 129 Table I Percentage germination of pollen grains in groups of different size. Number of grains in group Total number of grains 1 ······························ 467 2 426 3 462 4 320 5 375 6 240 7 ···································-245 8 192 9 207 10 160 11-19 incl. 707 3801 Literature Germinated number % 6 1.3 57 13.4 104 22.6 123 38.5 188 50.0 136 56.8 145 59.2 132 68.8 122 59.0 117 73.6 548 72.4 1678 44.2 Cited Allee, W. C. 1931. Animal aggregations. University of Chicago Press, Chicago. Allee, W. C. and G. Evans. 1937. Further studies on the effect of numbers on the rate of cleavage in eggs of Arbacia. J. Cell. and Comp. Physiol. 10: 15-28. Beams, H. W. and R. L. King. 1944. The "negative group effect" in the pollen grains of Vinca rosea. J. Cell. and Comp. Physiol. 23: 39-45. Beck, W. A. and R. A. Joly. 1941. Some growth phenomena in cul-tured pollen tubes. Trans. Am. Mier. Soc. 60: 149-162. Brink, R. A. 1924. The physiology of pollen IV. Am. J. Bot. 11: 417-436. Harrison, R. G. 1928. On the status and significance of tissue culture. Arch. f. exp. Zellforsch. Bd. 6: 24-27. Merwin, R. 1945. Some group effects on the rate of cleavage and early development of frog eggs {Rana pi pi ens). Physiol. Zool. 18: 16-34, 3Beams and King: Pollen Germination in Relation to Group SizePublished by UNI ScholarWorks, 1947

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Pollen Germination in Relation to Group Size

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University of Northern Iowa