ORGANIC STRUCTURE. 97 



being- furnished with latent buds or viviparous prin- 

 cipals, whence new plants are produced, while the 

 latter are not. The JPcebnia furnishes an example 

 of the tuberous root. 



Fig. 30. 



Ligneous or woody stems are composed of several 

 distinct members which may be described in the 

 order of their position, beginning with 



The Pith. This member occupies the centre of 

 the stem, and constitutes the principal part of the 

 bulk of the seedling, and of every young shoot. It 

 is more or less filled with spongeous, cellular matter, 

 divided into large longitudinal locoments, easily per- 

 meableby the fluids of the system. (Fig. 1.) Although 

 its apparent use is to strengthen the young shoot, and 

 to act as a duct or reservoir of moisture, its structure 

 and numerous hollow spaces show, that it may be 

 also a chamber for containing an elastic gas, which 

 when acted on by the heat of the air, must necessa- 

 rily assist to distend and elongate the sheath in which 

 it is contained. There seems to be no action in the 

 pith (except as a duct) after the first year, for as it 

 increases in age it decreases in volume ; and in old 

 stems becomes almost obliterated. A thin sheath 



