M-MTION. | 1MTH. IV? 



CHAPTER VI. 



OF THE STEM AND THE ORIGIN OF WOOD. 



THE general purpose of the stem is, to bear the leaves and 

 other appendages of the axis aloft in the air, so that they may 

 be freely exposed to light and atmospheric action ; to convey 

 fluids from the root upwards, and from above downwards ; 

 and, if woody, to store up a certain portion of the secretions 

 of the species either in the bark or in the heartwood. 



Various notions have from time to time been entertained 

 about the PITH. The functions of brain, lungs, stomach, 

 nerves, spinal marrow, have by turns been ascribed to it. 

 Some have thought it the seat of fecundity, and have believed 

 that fruit trees deprived of pith became sterile ; others sup- 

 posed that it was the origin of all growth ; and another class 

 of writers have declared that it is the channel of the ascent 

 of sap. 



It is probable that its real and only use is, to serve, in the 

 infancy of a plant, for the reception of the sap upon which 

 the young and tender vessels that surround it are to feed 

 when they are first formed ; a time when they have no other 

 means of support. In the winter it is often rich in starch, 

 which changes into gum in the progress of vegetation, and, 

 in a state of solution, passes from the pith into the young 

 organs in communication with the pith. It sometimes con- 

 tains resinous and other insoluble matters, which has led 

 Professor Morren to regard it as a " species of cloaca where 

 substances henceforward useless accumulate." 



Dutrochet considers it to act not only as a reservoir of 

 nutriment for the young leaves, but also to be the place in 

 which the globules which he calls nervous corpuscles are 

 formed out of the elaborated sap (U Agent Immediat, &c. 



