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CHAPTER III. 
OF THE STEM AND THE ORIGIN OF WOOD. 
The general purpose of the stem is to bear the leaves and 
and Other appendages of the axis aloft in the air, so that they 
maybe freely exposed to lightand 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, we cannot say observers, have declared that it was 
the channel of the ascent of sap. It is, however, no part of 
the plan of this work to refute these and similar exploded 
speculations. 
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. 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 cor- 
puscles, are formed out of the elaborated sap. [U Agent 
Immediate &c., p. 44, &c.) ; and Braschet imagines it and its 
processes to constitute the nervous system of plants. 
The MEDULLARY SHEATH sccms to perform a most import- 
ant part in the economy of plants ; it diverges from the pith 
whenever a leaf is produced ; and, passing through the petiole, 
ramifies among the cellular tissue of the blade, where it 
