84 HYPOCOTYLS, EPICOTYLS, ETC., CHAP. II. 



were placed in a horizontal position in damp air in the 

 dark, and in the course of 4 hours they again became 

 curved vertically downwards, having passed through 

 90 in this time. But their sensitiveness to geotropism 

 lasts for only 2 or 3 days; and the terminal part 

 alone, for a length of between *2 and '4 inch, is thus 

 sensitive. Although the petioles of our specimens 

 did not penetrate the ground to a greater depth than 

 about J inch, yet they continued for some time to grow 

 rapidly, and finally attained the great length of about 

 3 inches. The upper part is apogeotropic, and there- 

 fore grows vertically upwards, excepting a short 

 portion close to the blades, which at an early period 

 bends downwards and becomes arched, and thus 

 breaks through the ground. Afterwards this portion 

 straightens itself, and the cotyledons then free them- 

 selves from the seed-coats. Thus we here have in 

 different parts of the same organ widely different kinds 

 of movement and of sensitiveness ; for the basal part 

 is geotropic, the upper part apogeotropic, and a portion 

 near the blades temporarily and spontaneously arches 

 itself. The plumule is not developed for some little 

 time ; and as it rises between the bases of the parallel 

 and closely approximate petioles of the cotyledons, 

 which in breaking through the ground have formed an 

 almost open passage, it does not require to be arched and 

 is consequently always straight. Whether the plumule 

 remains buried and dormant for a time in its native 

 country, and is thus protected from the cold of winter, 

 we do not know. The radicle, like that of the Megar- 

 rhiza, grows into a tuber-like mass, which ultimately 

 attains a great size. So it is with Ipomoea pandurata, 

 the germination of which, as Asa Gray informs us, 

 resembles that of I. leptopliylla. 



The following case is interesting in connection with 



