142 



LECTURE IX. 



a thin clothing to the cell-wall. The remaining space is filled with watery sap; 

 and the parenchyma cells may contain in addition the most various products of 

 assimilation and metabolism. It is chiefly in these cells that such substances are 

 stored temporarily, or even up to the next period of vegetation and longer, 

 to be dissolved and used up later for the purposes of growth. In this form we 

 find the parenchymatous tissue as a succulent envelope surrounding the axial 

 strand of the young root, as pith, and primary cortex ; and to a certain extent as 

 the mass surrounded by the epidermis, and filling up the parts between the vascular 

 bundles, in the shoot-axes and leaf-stalks, and in the leaves themselves, as well 

 as in the coverings of the fruit. Even the endosperm of seeds and the coty- 

 ledons of embryos may be classed under this head. 



However, this parenchymatous cell-tissue is by no means the only form 

 in the system of the fundamental tissue; for the parenchyma itself undergoes 



FIG. 149. — A portion of the transverse section of the flower stem of Fteiticttliitn ojgicinalet slightly magnified. 

 t epidermis ; hi wood ; k resin passages. All the rest is fundamental tissue — m parenchyma of the pith ; r paren. 

 chyma of the cortex ; ch parenchyma containing chlorophyll ; c coUenchyma. 



more or less extensive differentiations, without losing its essential peculiarities, and, 

 very frequently, cells and forms of tissue of the most various kind appear in it 

 besides. 



A differentiation within the parenchymatous fundamental tissue is very gene- 

 rally exhibited, firstly, in so far that the intercellular spaces towards the epidermis, 

 as well as at the borders of the vascular bundles, disappear, and th? sizes of the 

 cells, and especially- their diameters,- diminish; and," generally, alterations in the 

 thickness, substance, and pitting of the walls occur also. These changes not rarely 

 proceed so far that the layers pf tissue concerned entirely lose their parenchymatous 

 character : they then have different names assigned to them. 



Thus, it is a very common phenomenon that beneath the epidermis of the 

 shoot-axes, petioles and thicker leaf-ribs of dicotyledonous plants, the cells of- the 

 fundamental tissue are developed as CoUenchyma (Fig. 150); the longitudinal 



