20S 



VEGETABLE 



gioles, greatly magnified ; <z, a, the spongioles in ^f- 23* 

 the shriveled state, lying close to the fibril ; b. b, b, 

 the same expanded, and projecting from the fibril, 

 after it is placed in water, showing their natural 

 positions in the earth. Some families of plants, 

 instead of this capillary fibril, are furnished with 

 spongioles on the rootlets themselves. 



From some experiments of M. Dutrochet, it ap- 

 pears that these spongioles are the organs by which 

 the plants absorb their entire nourishment from the 

 earth, and that they imbibe the fluids through the cuticle, be- 

 ing aided in the process by electrical influence. 



Cambium. The general structure of the wood, bark, and 

 trith, have been described, and we have seen that a concentric 

 circle of wood is every year added to the diameter of grow- 

 ing phaenogamous plants, and also, that a new layer of bark 

 is annually produced on the inside of the old, and by which 

 the latter is forced outwards and thickened. The functions 

 of the plant are chiefly carried on by these new parts, since 

 the interior of the tree may be entirely wanting, and still vig- 

 orous new limbs are every year produced. 



Now both the bark and the wood are formed by a gelatinous 

 secretion, called Cambium, which appears to be deposited 

 partly from the vessels of the bark, and partly from the me- 

 dullary rays between the wood and the bark. This juice is 

 however prepared in the leaves by the elaboration of the sap, 

 and then transmitted through the vessels of the bark, to be 

 converted into solid matter on every part of the plant. That 

 the matter which increases the plant is chiefly prepared in 

 the leaves, seems to be proved by the circumstance that when 

 the leaves of a tree, or shrub, are destroyed in the spring, 

 little or no new wood is formed that season, Fi =- 236 - 



and if the bark of the trunk of a tree be 

 gnawed off by animals, or otherwise removed, 

 the portion of the tree above it continues to 

 grow in diameter, because it is supplied with 

 the returning sap, which has been elaborated 

 by the leaves, while the portion below the 

 wounded bark ceases to grow because all 

 communication between it and the leaves is 

 cut off. This is represented by Fig. 236, 

 where a, arid b, show the increasing, and 

 stinted portions of the trunk, respectively. 



