CHAP. II BREAKING THROUGH THE GROUND. 85 



the root-like nature of the petioles. The radicle of a 

 seedling was cut off, as it was completely decayed, 

 and the two now separated cotyledons were planted. 

 They emitted roots from their bases, and continued 

 green and healthy for two months. The Wades of 

 both then withered, and on removing the earth the 

 bases of the petioles (instead of the radicle) were 

 found enlarged into little tubers. Whether these 

 would have had the power of producing two in- 

 dependent plants in the following summer, we do not 

 know. 



In Quercus virens, according to Dr. Engelmann,* 

 both the cotyledons and their petioles are confluent. 

 The latter grow to a length "of an inch or even 

 more ; " and, if we understand rightly, penetrate the 

 ground, so that they must be geotropic. The nutri- 

 ment within the cotyledons is then quickly transferred 

 to the hypocotyl or radicle, which thus becomes 

 developed into a fusiform tuber. The fact ot 

 tubers being formed by the foregoing three widely 

 distinct plants, makes us believe that their protection 

 from animals at an early age and whilst tender, is one 

 at least of the advantages gained by the remark- 

 able elongation of the petioles of the cotyledons, 

 together with their power of penetrating the ground 

 like roots under the guidance of geotropisrn. 



The following cases may be here given, as they bear 

 on our present subject, though not relating to seed- 

 lings. The flower-stem of the parasitic Lathrtea 

 squamaria, which is destitute of true leaves, breaks 

 through the ground as an arch ;t so does the flower- 



* ' Transact. St. Louis Acad. ground cannot fail to be great]} 



Science,' vol. iv. p. 190. facilitated by the extraordinary 



t The passage of the flower- quantity of water secreted afc thia 



etnm of the Lathrsea through the period of the year by the subter- 



