4 ON THE POTATOE PLANT. 



influenced by the external agency of heat, light, and elec- 

 tricity, undergoes various changes essential to the vitality 

 of the plant. 



(15.) These changes can only take place when the sap 

 is in proper relation to the cellular tissue, and both are 

 acted upon in a proper manner by heat and light. Upon 

 these changes depends the life of the plant ; and the moment 

 these changes, from any cause, cease, other changes, 

 those of putrefaction, ensue ; the plant loses its organic 

 existence, and is obedient to the laws which govern inor- 

 ganic bodies. 



(16.) Perhaps one of the most curious facts connected 

 with vegetable physiology is, that a whole plant must not 

 be subjected equally to these external forces one part 

 must be subjected to the light more than another ; and 

 this singular circumstance gives us the idea of a root and 

 leaf, the one creeping into dark and moist recesses, and 

 the other seeking the all-important influence of the sun's 

 rays. 



(17.) The starch deposited in the dilated stem or tuber, 

 is, doubtless, produced by the leaves, and acts as a source 

 or store-house of nutrition, from which future fibre may be 

 formed. It is not itself to be regarded as a perfect mate- 

 rial, but as a temporary substance deposited for the future 

 exigences of the plant. 



(18.) The mode in which plants procure their nitrogen- 

 ized substance, or albumen, does not appear quite clear. 

 It is possible that the nitrogen may become fixed from the 

 air, or it is possible that it may be derived from the earth 

 in which the plant grows. 



(19.) The mechanism whereby the fluid rises and the 

 solid part descends, is not certainly ascertained ; eventually, 



