MOVEMENTS OF THE COTYLEDONS 13 



is the arched hypocotyl. Once out of ground, the cotyledons 

 soon rise, until (at E) they are again vertical, but with the 

 other end up from that which stood highest in C. Then the 

 two cotyledons separate until they once more lie horizontally, 

 pointing away from each other. 



Whether the first part of the seedling to emerge from the 

 ground is a pointed, rolled-up leaf, as in Indian corn, or the hypo- 

 cotyl arch, as in Figs. 2 and 8, the result is to force the earth 

 aside without injury to the plumule or the cotyledons. 



15. What pushes the cotyledons up? A very little study of 

 any set of squash seedlings, or even of Fig. 8, is sufficient to 

 show that the portion of the plant where roots and hypocotyl 

 are joined neither rises nor sinks, but that the plant grows both 

 ways from this part (a little above r' in Fig. 8, A and B). It is 

 evident that as soon as the hypocotyl begins to lengthen much 

 it must do one of two things : either push the cotyledons out 

 into the air or else force the root down into the ground as one 

 might push a stake down. The plantlet, in passing from the 

 stage shown at A to that of B and of C, develops many lateral 

 roots, thus making it harder and harder for the root to be thrust 

 bodily downward. 



16. Getting rid of the seed coats. In seeds with thin coats 

 the latter usually burst open irregularly and allow the opening 

 cotyledons to escape. But in seeds with as thick a testa as that 

 of the squash, and still more in the case of nuts, the cotyledons 

 find their way out through a slit, or opening, which appears in 

 a definite part of the seed. If for any reason the seed coat does 

 not open, the embryo cannot grow. In many cases the moisture 

 and freezing and thawing of a winter in the earth are almost 

 essential to germination, and some seeds grow more promptly if 

 they have been scorched by fire, or if they are cracked open 

 before planting. 



17. Discrimination between root and hypocotyl. It is not 

 always easy to decide by their appearance and behavior what 

 part of the seedling is root and what part is hypocotyl. In a 



