70 OKGANOGRArHT. 



the base of the upper surface of the leaf with the stem). The 

 presence of leaves with leaf-buds in their axils is therefore the 

 essential characteristic of a stem, in contradistinction to a root 

 in which such organs are absent. All Flowering Plants, from 

 the mode in which their axis is developed in the embryo (p. 

 68), must necessai-ily have a stem, although such stem may 

 be very short. Those which have this organ clearly evident 

 are called caulescent, Avhile those in which it is very short 

 or inconspicuous are termed acaulescent or stemless. In FloAver- 

 Icss Plants the stem is not necessarily present; thus it is absent 

 in all Thallophytes. 



1. Inteknal Structure of the Stem in general. — The 

 simplest form of stem consists merely of parenchymatous 

 cells. An example of such a stem may be seen, with few 

 exceptions, in the Mosses and Liverworts. Such a structure 

 however would be unsuited to plants in which great strength is 

 required, and we accordingly find that in all plants above the 

 Mosses the stem is made up i)artly of parenchymatous cells, and 

 partly of wood-cells and vessels of ditlerent kinds, by whicli the 

 requisite strength and toughness are produced. In these stems 

 tlicrefore we distinguish two systems as already noticed (p. 38), 

 namely, a Parenchymatous or common cellular si/stem, and a Fibro- 

 vascultir. The jxtrenchijmatous system grows in any direction 

 according to circumstances, either longitudinally, by which the 

 stem is increased in length, or horizontally, by which it is in- 

 creased in diameter. Tlie fibro-vascular system only grows 

 longitudinally, and thus forms cords or bundles Avhich are dis- 

 tributed vertically in the midst of the parenchymatous. The 

 parenchymatous system is therefore also termed the horizontal 

 system of the stem, while the Jihro-va^cular is likewise called the 

 lonf/iti/dinal or vertical system. 



The differences Avhich are found to exist in the internal 

 structure of the stem of ])lants, are in a great measure owing 

 to the different ways iii which the fibro-vascular system is dis- 

 tributed in tlie parenchymatous. All these modifications may 

 be. in their essential i)articulars, reduced to three great classes, 

 two of wliich are found in Flowering Plants, and one in Flower- 

 less. As illustrations of the two former we may take an Oak 

 and a Palm stem ; of the latter, that of a Tree-fern. 



Upon making a transverse section of the Oak (Ji(j. 163) we 

 ol)serve that the two systems of which the stem is c()mj)osed are 

 so arranged as to exhibit a distinct separation of ])arts. Thus 

 we have a central one, /«, called the pith; an external one, c e, or 

 harh ; :in intermediate, r, orz/vW, dis]>erscd in concentric layers; 

 and little rays, /;, eonneeting the jiith and the bark, termed 

 mediiltary rays. Such a stem grows in diameter by constant 

 additions of new matter on the outside of its wood, and hence 

 it is called Exogenous (from two Greek words signifying outside 



