Chap. XII. CONCLUDING EEMAEKS. 557 



Ipomcea leptophylla and pandurata, and of Quercus 

 virens, is connected with the burying of the tuber-like 

 roots, which at an early age are stocked with nutri- 

 ment ; for in these plants it is the petioles of the 

 cotyledons which first protrude from the seeds, and 

 they are then merely tipped with a minute radicle and 

 hypocotyl. These petioles bend down geotropically 

 like a root and penetrate the ground, so that the true 

 root, which afterwards becomes greatly enlarged, is 

 buried at some little depth beneath the surface. Gra- 

 dations of structure are always interesting, and Asa 

 Gray informs us that with Ipomcea Jalappa, which 

 likewise forms huge tubers, the hypocotyl is still of 

 considerable length, and the petioles of the cotyledons 

 are only moderately elongated. But in addition to the 

 advantage gained by the concealment of the nutritious 

 matter stored within the tubers, the plumule, at least 

 in the case of Megarrhiza, is protected from the frosts 

 of winter by being buried. 



With many dicotyledonous seedlings, as has lately 

 been described by De Vries, the contraction of the 

 parenchyma of the upper part of the radicle drags the 

 hypocotyl downwards into the earth ; sometimes (it is 

 said) until even the cotyledons are buried. The hypo- 

 cotyl itself of some species contracts in a like manner. 

 It is believed that this burying process serves to 

 protect the seedlings against the frosts of winter. 



Our imaginary seedling is now mature as a seedling, 

 for its hypocotyl is straight and its cotyledons are 

 fully expanded. In this state the upper part of the 

 hypocotyl and the cotyledons continue for some time 

 to circumnutate, generally to a vrade extent relat vely 

 to the size of the parts, and at a rapid rate. But 

 seedlings profit by this power of movement only when 

 it is modified, especially by the action of light and 



