Chap IL HYPOCOTYLS AND EPICOTYLS. 77 



earth on all sides, with a force of above 8 pounds in 

 one case, of 3 pounds in another case. It was impos- 

 sible to decide whether the actual apex exerts, relatively 

 to its diameter, the same transverse strain as the parts 

 a little higher up ; but there seems no reason to doubt 

 that this would be the case. The growing part there- 

 fore does not act like a nail when hammered into a 

 board, but more like a wedge of wood, which whilst 

 slowly driven into a crevice continually expands at 

 the same time by the absorption of water; and a 

 wedge thus acting will split even a mass of rock. 



Manner in which Hypocotyls, Epicotyls, &g., rise up 

 and break through the ground. — After the radicle has 

 penetrated the ground and fixed the seed, the hypo- 

 cotyls of all the dicotyledonous seedlings observed by 

 us, which lift their cotyledons above the surface, break 

 through the ground in the form of an arch. When 

 the cotyledons are hypogean, that is, remain buried in 

 the soil, the hypocotyl is hardly developed, and the 

 epicotyl or plumule rises in like manner as an arch 

 through the ground. In all, or at least in most of such 

 cases, the downwardly bent apex remains for a time 

 enclosed within the seed-coats. With Corylus avel- 

 lena the cotyledons are hypogean, and the epicotyl 

 is arched; but in the particular case described in 

 the last chapter its apex had been injured, and it 

 grew laterally through the soil like a root; and in 

 consequence of this it had emitted two secondary 

 shoots, which likewise broke through the ground as 

 arches. 



Cyclamen does not produce any distinct stem, and 

 only a single cotyledon appears at first ; * its petiole 



• This Is the conclusion arrived considered by other botanists as 



at by Dr. H. Gressner ('Bot. the first true leaf is really the 



Zeitung,' 1874, p. 837), who second cotyledoTi, which is greatly 



maintains that what bas been delayed in its development. 



