CHAP. II. HYPOCOTYLS, ETC., WHILST ARCHED. 101 



the pressure of the surrounding earth be artificially 

 removed, the arch immediately begins to straighten 

 itself. This no doubt is due to growth along the 

 whole inner surface of both legs of the arch; such 

 growth being checked or prevented, as long as the two 

 legs of the arch are firmly pressed together. When the 

 earth is removed all round an arch and the two legs 

 are tied together at their bases, the growth on the 

 under side of the crown causes it after a time to 

 become much natter and broader than naturally 

 occurs. The straightening process consists of a mo- 

 dified form of circumnutation, for the lines described 

 during this process (as with the hypocotyl of Brassica, 

 and the epicotyls of Yicia and Corylus) were often 

 plainly zigzag and sometimes looped. After hypo- 

 cotyls or epicotyls have emerged from the ground, 

 they quickly become perfectly straight. No trace is 

 left of their former abrupt curvature, excepting in the 

 case of Allium cepa, in which the cotyledon rarely 

 becomes quite straight, owing to the protuberance 

 developed on the crown of the arch. 



The increased growth along the inner surface of the 

 arch which renders it straight, apparently begins in 

 the basal leg or that which is united to the radicle ; 

 for this leg, as we often observed, is first bowed back- 

 wards from the other leg. This movement facilitates 

 the withdrawal of the tip of the epicotyl or of the 

 cotyledons, as the case may be, from within the seed- 

 coats and from the ground. But the cotyledons often 

 emerge from the ground still tightly enclosed within 

 the seed-coats, which apparently serve to protect them. 

 The seed-coats are afterwards ruptured and cast off by 

 the swelling of the closely conjoined cotyledons, and not 

 by any movement or their separation from one another. 



Nevertheless, in some few cases, especially with the 



