Moore: Self-Sterilitv 



207 



action of the stij^mia to a pollen tube 

 from a cross-pollination may be different 

 from that to a pollen tube from a 

 self-pollination. He states that "all 

 gametes having in their hereditary con- 

 stitution something different from that 

 of the cells of the mother plant, how- 

 ever, can provoke the proper secretion 

 to stimulate the pollen tube growth, 

 reach the ovary before the flower wilts, 

 and produce seeds." From this it may 

 be inferred that there may be an enzyme 

 in the pollen grain that in a cross- 

 pollination is able to induce the stigma 

 to excrete a stimulating substance so 

 that the pollen tube is able to grow. 

 In a self-pollination this enz>Tne is not 

 able to act. However, if this were the 

 case, when a few cross-pollen grains 

 were placed on a self-pollinated stigma, 

 they would be expected to germinate and 

 cause the stigma to produce the stimu- 

 lating substance. Thus the pollen tubes 

 from the self-pollination would also 

 benefit by the stimulating influence and 

 should be able to grow and bring about 

 fertilization. However, the work with 

 alsike clover does not support this hy- 

 pothesis. It must be assumed that if any 

 pollen tubes were able to grow that it 

 was due to this stimulating influence. 

 As a fact, three pollen tubes from a self- 

 pollination of alsike clover grew through- 

 out the length of the style while the 

 other tubes remained short. If this 

 enzyme hypothesis were to be accepted, 

 the same stimulus which made the three 

 pollen tubes grow should also have 

 made the other pollen tubes grow. 



EFFECT OF FOOD SUPPLY 



The second alternative may be taken, 

 namely, that the greater width of the 

 self-pollinated pollen tubes of Trades- 

 cantia is due to the fact that the food 

 supply is more favorable to the nourish- 

 ment of a self -pollen tube than it is to a 

 cross-pollen tube. On account of the 

 abundant food supply* the pollen tubes 

 did not lengthen, but grew wider since 

 they were in a very favorable medium. 

 By this hypothesis it is possible to ex- 

 plain most of the data here presented in 

 regard to self -sterility, and it is not 

 contradictory to any cytological evi- 

 dence. The Shirley poppy results 

 showed that a small proportion of the 



plants were probably cross-sterile. How- 

 ever, this is only reasonable since if a 

 large number of plants were used, a few 

 of them would be expected to have 

 nearly the same kind of food supply and 

 so be cross-sterile. East (1915, p. 

 81), working with Nicotiana, and Stout 

 (1916, p. 384) with dehor ittm intyhtts 

 also found a small percentage of plants 

 to be cross-sterile. 



Thus the h^'pothesis suggested by 

 Compton (1913, p. 200) that self-ster- 

 ility in plants may be analogous with 

 wheat which is immune to the rust 

 fungus, seems very probable. Briefly, 

 the main evidence favoring such an ex- 

 planation is as follows: (1) The primary 

 function of the pollen tube is to obtain 

 food. (2) The method of obtaining food 

 and manner of growth are similar to a 

 fungus. (3) Pollen tubes of self-iJollina- 

 tions of Tradescantia were about one- 

 third wider than those of cross-pollina- 

 tions. (4) If a number of pollen tubes 

 grow throughout the length of the 

 style, other pollen tubes in that style 

 may remain short. However, this ex- 

 planation is only tentative, and much 

 more data should be obtained before it 

 can be held that this is the correct 

 solution. 



BIBLIOGRAPHY 



Compton, R. H. Phenomena and Problems 

 of Self-sterility. New Phytologist, 12: 197-206. 

 1913. 



Correns, C. Selbststerilitat und Individual- 

 stoflfe. Festschr. d. mednat. Ges. 2. 84 Ver- 

 sam. deutsch. Naturf. u. Arzte. 1912. 



Coulter, J. M., Barnes, C. R., and Cowles, 

 H. C. A Textbook of Botany. Vol. i, 1-484. 

 American Book Co., New York, 1910. 



East, E. M. The Phenomenon of Self- 

 sterilit3^ TJie American Naturalist, 49: 76-87. 

 1915. 



East, E. M. An Interpretation of vSelf- 

 Sterility. Proc. Nat. Acad. Sc, 1: 95-100. 

 1915. 



Martin, J.N. The Phj^siology of the Pollen 

 of Trijoliitm pratense. Bot. Gaz., 56: 112-126. 

 1913. 



Martin, J. N. Comparative Morphology- of 

 some Leguminosae. Bot. Gas., 58: 154-167. 

 1914. 



Peters, Reg. W. Pollinating Fruit Trees. 

 JouRN.VL OF Heredity, 7: 365-369. 1916. 



Stout, A. B. Self- and Cross- Pollinations in 

 Cichorium inlyhiis with Reference to Sterility. 

 Memoirs of the New York Botanical Garden, 

 6: 333-454. 1916. 



Westgate, J. M., and Coe, H. S. Red Clover 

 Seed Production. U. S. D. A. Bulletin 289: 

 1-31. 1915. 



