The Orderly Cycles Pursued in Growth 355 



some kinds may remain, with vitality intact, for years and dec- 

 ades, and even a century, though not for the ages implied in the 

 current but groundless belief that genuine seeds from the wrap- 

 pings of mummies will germinate. In this condition, small and 

 light, and independent of external food or water supply, the seed 

 is capable of wide transport; and thus forms a natural stage 

 for dissemination, in adaptation to which its coats or neighboring 

 structures often develop wings, plumes, hooks, pulp and colors, as 

 we shall consider more fully in the following chapter. It is not of 

 course because the seed has these characters that it is utilized by 

 the plant as its dissemination stage, but it is rather because it has 

 been developed as the dissemination stage that it has these char- 

 acters. 



In following the sequence of cell divisions involved in these re- 

 sults one cannot but wonder what the nature of the controlling 

 power must be. Structurally considered, cell division can take 

 place just as well in one direction as another, yet in fact it takes 

 place in substantially the same directions as in preceding genera- 

 tions of embryos, directions which bring an adaptive result. 

 What is it which compels the developing egg-cell to form a line of 

 cells instead of a ball, and the initial cell to form a ball instead of 

 a line; which leads the ball to push out the two cotyledons in 

 definite places, and to make the hypocotyl and root in another? 

 In some way, it is certain, the control issues from the chromo- 

 somes, which alone hold the knowledge of how the former genera- 

 tions developed; but through what mechanism do they exert 

 their authority? This question, for the most part, we cannot yet 

 answer, but in some part we can; for it seems reasonably certain 

 that most of the changes consist in responses to stimuli, the nature 

 of which was explained in our chapter on Irritability. Perhaps 

 the pressure of the egg-cell against the end of the embryo-sac is 

 the stimulus which sends the suspensor developing as a single cell- 

 line in the opposite direction; perhaps the freer osmotic absorption 

 permitted by the arrival of the initial cell into the more fluid 



