24 VITAL ACTIONS. 



take their rise in the leaves, and which, thence pass- 

 ing downwards through the cellular tissue, are incor- 

 porated with the latter, to which they give the 

 necessary degree of strength and flexibility. In trees 

 and shrubs, they combine intimately with each other, 

 and so form what is properly called the wood and 

 inner bark ; in herbaceous and annual plants, they 

 constitute a lax fibrous matter. No woody matter 

 appears till the first leaf, or the seed-leaves, have 

 begun to act ; it always arises from their bases ; it 

 is abundant, on the contrary, in proportion to the 

 strength, number, and development of the leaves; 

 and in their absence is absent also. 



44. When woody matter , is first plunged into the 

 cellular tissue of the nascent stem, it forms a circle 

 a little within the circumference of the stem, whose 

 interior it thus separates into two parts ; namely, the 

 bark or the superficial, and the pith or the central, 

 portion ; or, in wliat are called Endogens, into a 

 superficial coating analogous to bark, and a central 

 confused mass of wood and pith intermingled. The 

 effect of this, in Exogens, is, to divide the interior of 

 a perennial stem into three parts, the pith, the wood, 

 and the bark. 



45. As the cellular tissue of the stem is not sensi- 

 bly lengthened more in one direction than in ano- 

 ther, and as it is the only kind of organic matter that 

 in stems increases laterally, it is sometimes con- 

 venient to speak of it under the name of the horizon- 

 tal system ; and, for a similar reason, to designate the 

 woody tubes which are plunged among it, and which 



