Chap. II. BEEAKING THROUGH THE GROUND. 83 



mules still enclosed within the tubes ; and he remarks 

 that if the plumules had been at once developed and 

 had reached the surface (as occurred with our seeds 

 which were exposed to a high temperature), they 

 would surely have been killed by the frost. As it is 

 they lie dormant at some depth beneath the surface, 

 and are thus protected from the' cold ; and the root- 

 hairs on the petioles would supply them with sufficient 

 moisture. We shall hereafter see that many seedlings 

 are protected from frost, but by a widely different 

 process, namely, by being drawn beneath the surface 

 by the contraction of their radicles. Vv'^e may, how- 

 ever, believe that the extraordinary manner of germi- 

 nation of Megarrhiza has another and secondary 

 advantage. The radicle begins in a few weeks to 

 enlarge into a little tuber, which then abounds with 

 starch and is only slightly bitter. It would therefore 

 be very liable to be devoured by animals, were it not 

 protected by being buried whilst young and tender, at a 

 depth of some inches beneath the surface. Ultimately 

 it grows to a huge size. 



Ipomoea leptopliylla. — In most of the species of this 

 genus the hypocotyl is well developed, and breaks 

 through the ground as an arch. But the seeds of the 

 present species in germinating behave like those of 

 Megarrhiza, excepting that the elongated petioles of 

 the cotyledons are not confluent. After they have 

 protruded from the seed, they are united at their 

 lower ends with the undeveloped hypocotyl and un- 

 developed radicle, which together form a point only 

 about -1 inch in length. They are at first highly 

 geotropic, and penetrate the ground to a depth of 

 rather above half an inch. The radicle then begins 

 to grow. On four occasions after the petioles had 

 grown for a short distance vertically downwards, the-v 



