232 INTRODUCTION. [part n. 



the spiral vessels will be seen distinctly with 

 the naked eye ; as though very fine, they are 

 sufficiently strong to sustain the weight of the 

 lower part of the leaf for a short time. Now 

 these vessels are much more conspicuous in 

 some plants than in others ; and in some of the 

 inferior plants, such as lichens and fungi, they 

 are wanting altogether. Their presence or 

 absence has therefore been chosen to mark the 

 two great divisions of the Natural System ; 

 the Vasculares being those plants which have 

 both vascular and cellular tissue, and the Cellu- 

 lares being those which have cellular tissue only. 

 The Vasculares are much more numerous 

 than the Cellulares ; and they are redivided into 

 subclasses, which it also requires the aid of vege- 

 table physiology to explain. All plants, when 

 in a growing state, require the moisture taken 

 up by their roots to be elaborated, and mixed 

 with air in the leaves before it becomes sap ; 

 that is, before it is fit to contribute to their 

 nourishment. Now, when a seed begins to ger- 

 minate, its root descends into the ground and 

 its plumule, or ascending shoot, rises upwards ; 

 but the leaves folded up in the latter are too 

 weak and tender to elaborate the sap ; and 

 besides, they cannot act till they are fully 

 expanded, and they want nourishment to give 

 them strength to unfold; the roots are also not 



