256 



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 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, 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 woi'k 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 

 Immediat, &c., p. 44, &c.) ; and Braschet imagines it and its 

 processes to constitute the nervous system of plants. 



The MEDULLARY SHEATH secms to pei'fomi 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 



