rORK. STEMS OF ENDOGENS. 



41. The external layer of cellular tissue constitutes the epi- 

 dermis, and is what botanists term the herbaceous envelope 

 of the bark. In the course of the growth of the subjacent parts, 

 it soon becomes strongly compressed, and at a certain epoch, we 

 see it crack and tear in flexible laminae, or detach itself in scales 

 or patches ; the neighbouring cortical layers undergo the same al- 

 terations, and when the part of the bark thus modified has been 

 raised up, the laminse of cellular tissue thus exposed becomes for 

 a brief period a kind of epidermis, until it is itself in turn detached. 

 For this reason the thickness of the bark is never very considera- 

 ble, and its surface is continually renewed. In some plants the 

 herbaceous layer becomes very much developed, and the portion 

 of bark that is thus separated is of sufficient consistence and thick- 

 ness to be very useful to us in the arts. Cork, for example, is 

 only the superficial part of the bark of a particular species of oak, 

 quercus robur, which detaches itself from the liver every 

 eight or nine years, and it may be removed more frequently with- 

 out any danger of destroying the tree. 



42. Bark often contains, in its interior, cavities which are 

 reservoirs of proper juices, and, in particular, those called the 

 vessels of the latex. 



Structure of the Stem of Endogenous Plants. 



43. The stem of these plants, 

 that of a palm, for example 

 (Jig. 14), is formed of a consi- 

 derable mass of cellular tissue, 

 analogous to pith, through which 

 penetrate bundles of fibres in 

 various ways, but never forming 

 concentric layers, as in the ex'o- 

 genous plants. Each of these 

 fibres is composed of elongated 

 cellules, of large dotted vessels, 

 of tracheae, of proper vessels, and 

 of polyhedral cells; they are 

 closer together near the centre 

 of the stem than towards its cir- 

 cumference, and their superior 

 extremity is abruptly curved out- 

 wards to be continued into the 



a b 



14. SECTION OF AN ENDOGEN. 



Explanation of Fig. 14. Section of the stem of an endogenous plant, 

 l a palm); a. cellular tissue ; b. fibres; c. external pellicle. 



41. What is epidermis ? How is it formed ? What is cork ? 



42. What does bark contain ? 



43. What is the structure of the stem in endogenous plants ? 



