33 THE sTE:\r. 



mildest mucilage of the Ulmus fulva and Bene plant, to the 

 most powerful poisons of the Daphne Mezerium, and of those 

 yielding the Hydrocyanic or Prussic Acid. Numerous ex- 

 amples might be added of the important productions of this 

 organ, but they will be reserved for a future section. 



43. The functions of the bark taken as a whole, seem to 

 be the protection of the newly formed wood, the secretiiig of 

 various products, and forming a channel through which the 

 descending elaborated sap may pass to the various parts of 

 the stem. The functions of the cellular integument and liber 

 separately considered have not been determined. There ap- 

 pears to be plausibility in the conjecture, that the cellular 

 part of the bark, being deposited first, acts the same part in 

 the formation of the liber, that the pith performs in a newly 

 formed branch, that of affording nourishment, if it does not 

 act some part in generating the fibres themselves. If, as has 

 been supposed, the cellular system is the generating appa- 

 ratus of vegetables — and that it is in some cases, we have the 

 best evidence — will it not afibrd a probable reason for the al- 

 ternate layers of the cellular and vascular tissues by suppo- 

 sing that the cellular tissue being first deposited, then acts 

 as the generator of the fibrous tissue of the liber ! 



Section 3. — The Stem, 



44. The Stem is the ascending axis of the plant. The 

 descriptions given in the pieceding sections on the cuticle and 

 bark, will apply to them in all cases where these organs are 

 found ) but no such general description can be given of the 

 stem. There are three distinct types of this organ, each of 

 which necessarily requires a distinct description. First, Exo- 

 genous stems which increase by external layers of woody 

 substance, and are covered with bark, and enclose a pith in 

 their center. The term is derived from two Greek words, 

 Exo outwardly, and Gene I produce. The term was given by 

 De Candolle to designate all those plants, which increase in 

 diameter by the addition of external layers, and includes all 

 the trees and shrubs of the Temperate zone, and all plants 

 which have true bark and reticulately veined leaves, whether 

 woody or not. If we take a stem of the oak, for instance, 

 and make a transverse section of it, we observe that the woody 

 part of it is composed of concentric layers of hard woody 

 substance, which consists of vascular tissue and woody fibre, 

 closely united. These layers are partially separated from 

 each other by a more porous, and of course, less dense layer. 



