CONDITIONS ESSENTIAL TO NUTRITION. 137 



serve, when accompanied by an azotized substance, 

 to sustain the embryo, until its first organs of nutri- 

 tion are unfolded. The nutrition of a foetus and de- 

 velopment of an egg proceed in a totally different 

 manner from that of an animal which is separated 

 from its parent ; the exclusion of air does not en- 

 danger the life of the foetus, but would certainly 

 cause the death of the independent animal. In the 

 same manner, pure water is more advantageous to 

 the growth of a young plant, than that containing 

 carbonic acid, but after a month the reverse is the 

 case. 



The formation of sugar in maple-trees does not 

 take place in the roots, but in the woody substance 

 of the stem. The quantity of sugar in the sap aug- 

 ments until it reaches a certain height in the stem 

 of the plant, above which point it remains stationary. 



Just as germinating^ barley produces a substance 

 which, in contact with starch, causes it to lose its 

 insolubility and to become sugar, so in the roots of 

 the maple, at the commencement of vegetation, a 

 substance must be formed, which, being dissolved in 

 water, permeates the wood of the trunk, and con- 

 verts into sugar the starch, or whatever it may be, 

 which it finds deposited there. It is certain, that 

 when a hole is bored into the trunk of a maple-tree 

 just above its roots, filled with sugar, and then closed 

 again, the sugar is dissolved by the ascending sap. 

 It is further possible that this sugar maybe disposed 

 of in the same manner as that formed in the trunks ; 

 at all events it is certain, that the introduction of it 

 does not prevent the action of the juice upon the 

 starch, and since the quantity of the sugar present is 

 now greater than can be exhausted by the leaves 

 and buds, it is excreted from the surface of the 

 leaves or bark. Certain diseases of trees, for exam- 

 ple that called honey-dew, evidently depend on tbe 

 want of the due proportion between the quantity of 

 the azotized and that of the unazotized substances 

 which are applied to them as nutriment. 



12* 



