356 The Living Plant 



central part of the embryo-sac is the stimulus which sends this cell 

 developing into a regular mul-ticellular ball; perhaps the beginning 

 of pressure on this ball as its expansion brings it against the 

 protoplasmic lining of the embryo-sac, is the stimulus which sets 

 the cotyledons developing at their definite places, which places 

 themselves may be fixed by the positions of least pressure; 

 perhaps the contact of these growing cotyledons with one another 

 is the stimulus which starts the development of the plumule be- 

 tween them, and starts also the extension to form hypocotyl and 

 root in the other direction. Maybe, or probably, I am wrong as 

 to the details of these stimuli, but if it is not these it is some others 

 of similar sort; and hi any case my speculations illustrate the 

 principle of the matter. The idea is confirmed by the fact that 

 there is one case of growth stimulation of whose nature we are 

 reasonably certain. The reader will recall that the stimulus given 

 by the fusion of the second nucleus of the pollen-tube with the 

 nucleus of the embryo-sac starts the development of the endo- 

 sperm (page 299) ; and this, or some other phase of fertilization, 

 is the stimulus which starts not only the hardening of the seed 

 coats and the development of other typical seed features, but also 

 the many large processes involved in the formation of the fruit. 

 It is a fact that ordinarily neither endosperm, seed coats, nor fruit, 

 develop unless fertilization is effected, an arrangement that is 

 obviously adaptive, since without fertilization they would all of 

 them be useless, and a wasteful drain on the plant. This kind of 

 "linking up" of the processes together through the connection of 

 stimuli is believed to be representative of the method whereby 

 the development of plants, and animals too, is kept in harmonious 

 and continuous progress. It is essentially the same method 

 as that by which the parts of a complicated machine are kept 

 working effectively together, each special part of the mechan- 

 ism being geared or connected to some of the others in such man- 

 ner that the movement of the mass as a whole compels each part to 

 perform its destined office at just the right moment and place. 



