142 PLANT PHYSIOLOGY 



prepotence (in the sense in which these terms are used horticulturally) 

 in higher plants phenomena which may not be explained with 

 present information. It has been found that tomato and tobacco 

 fruits are sometimes formed without pollination; and the same is 

 true of other plants. In certain cucurbits the act of pollination 

 seems to afford a stimulus for the development of the fruit, even 

 the dead pollen serving to call forth this response. Under such cir- 

 cumstances it may well be that other chemical stimuli may produce 

 the same effect. On the whole, there are no more interesting pro- 

 blems in physiology than those relating to pollination, the penetra- 

 tion of the pollen tube, and conditions of fertilization. Many phases 

 of these problems have thus far been studied by gardeners and 

 horticulturists alone. 



In this connection may be mentioned another fusion phenomenon 

 of physiological interest, that of double fertilization in the angio- 

 sperms. This fusion of the second sperm nucleus with the endo- 

 sperm nucleus (itself a compound of two nuclei of the gametic 

 groups) or with one of the polar nuclei, may have a special signifi- 

 cance, or it may be merely the expression of the fusion tendency 

 which has not been lost, although the function of the endosperm 

 nucleus may have undergone specialization. In the case of the 

 pine, it will be remembered that the second sperm nucleus fre- 

 quently undergoes division in the cytoplasm of the egg. What is 

 meant by the fusion of the gametes? This is always a fundamental 

 problem. It may be strictly a matter of the fusion of characters, or 

 it may further be a stimulus to embryonic growth. It is a remark- 

 able fact, however, that this stimulus to embryonic growth does not 

 merely involve the embryo itself. The limitations of the correlations 

 which seem to exist between the mere process of fertilization and 

 incitation to growth in the extra-carpellary structures are extremely 

 complex. On the other hand, the process of fusion is often imme- 

 diately followed by the resting period. 



It would be extremely well if further attention could be directed 

 to the matter of parthenogenesis in the higher plants. Except in 

 the case of Nathanson's studies upon Marsilia, little has been done 

 to indicate the conditions which may induce or which may tend 

 to induce this process. In recent years, artificial fertilization, or 

 stimulus to a certain growth of the egg in the lower animals, has 

 been effected by chemical agents, by changes in the density of the 

 solution, and by other means. This work has demanded world- wide 

 attention from animal physiologists. It has been too much neglected 

 from a botanical point of view, although the difficulties involved 

 in similar studies with plants would be, for the most part, immeas- 

 urably greater. Yet it is certainly possible to prosecute such studies 

 along the lines indicated. 



