262 TIMBERS AND THEIR USES 



behind the growing point, easily recognized by 

 the fact that its cells are elongated. This core is 

 known as the plerome, and by tracing it back- 

 wards to still older parts of the stem we shall 

 see that it forms the vascular system (wood 

 and blast). 



Surrounding th.e plerome, and distinguished by 

 its cells being divided both parallel and at right 

 angles to the exterior of the stem, is the peri- 

 blem, a tissue which gives rise to the cortex. 



. Outside the stem there is a single layer of cells, 

 with transverse walls which are thickened on 

 the exterior. This tissue is the dermatogen and 



■gives rise to the epidermis of the plant. It is 

 clear, therefore, that from the meristem of the 

 growing point tissues arise which become, 

 epidermis, cortex and vascular system. There 

 are other meristematic or dividing cells, but 

 they do not occur in the very young parts of 

 the plant and we shall treat of them later. 



Let us now take a cross section through a 

 stem a httle way behind the growing point, 

 but not so far that all the tissues are fully 

 developed. Our section, if we have made it 

 at the right place, will show us g, large number 

 of very thin- walled, comparatively big cells, 

 surrounded by a ring of almost egg-shaped 

 groups of cells so arranged that their more 

 pointed ends are directed towards the centre 



