12 INTRODUCTION. 



Others have a soft stem, which, after the fruit is perfected, 

 decays, and is named herbaceous. 



Others again have a hard -woody stem. Plants possessed of this 

 kind of stem are, when large, called Trees ; when small, Shrubs. 



In the woody stem there are distinguished the following parts: 

 the epidermis, or external thin filmy covering ; the cellular in- 

 tegument, a thin layer under the epidermis ; the outer bark, ge- 

 nerally of a brown or grey colour ; the inner bark, a whitish, soft, 

 and very flexible part, lying under the outer bark ; the alburnum, 

 or layers of young wood next to the inner bark ; the duramen or 

 hard wood, also in layers ; and lastly, the pith, a white, cellular, 

 spongy substance, occupying the centre of the stem. 



The stems of trees and shrubs growing in our climate are all of 

 this kind. They increase in size by the addition of a new layer 

 of wood every year to the alburnum, and of a thinner layer to the 

 inner bark. It is thus between the wood and the bark, that the 

 substance is formed, which, at first a clammy fluid, named Cam- 

 bium, is gradually organized, and converted into wood and bark. 

 Plants that increase in size in this manner, are called Exogenous, 

 that is, growing outwards, because their growth takes place near 

 the surface. But palms, and other trees of tropical countries, hav- 

 ing no distinction of parts into pith, woody layers, bark, and epi- 

 dermis, enlarge by the addition of fibres to their interior, and are 

 thus said to be Endogenous, or growing inwards. 



Herbaceous stems have also an epidermis, cellular substance, 

 fibrous and vascular tissue, and sometimes internal pith. 



The stem of a tree is called its Trunk ; the divisions of the stem 

 are the Branches and Twigs. 



Stems in general may be simple, having no divisions ; or 

 they may be branched. Of the latter there are several kinds. 



A Forked or Dichotomous Stem (Fig. 10.) is when it is regu- 

 larly and repeatedly divided into two, and a flower springs from 

 each fork. 



A Panicled Stem is when branches come off irregularly, and 

 are themselves irregularly divided, the ultimate divisions bearing 

 flowers. 



T-wo-ranked, when the branches spread in two opposite di- 

 rections. 



Four-ranked, when they spread in four directions. 



