MORPHOLOGY 



75 



red chromoplasts and the green chloroplasts. There is occasionally 

 found in the cell sap a yellow colouring matter known as xanthin ; 

 it is nearly related to xanthophyll, but soluble in water. The cell 

 sap also contains inorganic salts in solution, particularly nitrates, 

 sulphates, and phosphates. 



The Cell Wall. — At the growing points of plants the cells are 

 separated from one another only by extremely thin membranes or cell 

 walls. The rapid growth in length which sets in a short distance from 

 the growing point, as a result of the increase in the size of the cells, 

 must be accompanied by a corresponding growth in surface of the 



Fig. SI. — Strongly thickened cell 

 from the pith of ClenuUis vilalba. 

 m, Middle lamella ; i, intercel- 

 lular space ; t, pit ; w, pitted 

 transverse cell wall. (X300.) 



Fig. 82. — Part of a sclerenchy- 

 matous fibre from Viiico 

 major. The striatums of 

 the outer layers are more 

 apparent than those of 

 the inner layers. The 

 walls, as seen in optical 

 section, are also shown. 

 (X500.) 



cell walls. So long as this growth in surface continues, the cell walls 

 remain thin. After the cells have attained their ultimate size, the 

 growth in thickness of the cell walls then begins. Such thickened 

 cell walls are not, in most cases, homogeneous, but exhibit a stratified 

 appearance (Fig. 81), owing to the different refractive power of the 

 thickening layers. Treated with caustic potash, these different layers 

 appear as if composed of still thinner lamella?. In many cases the thick- 

 ening layers exhibit delicate striations in surface view. The striations 

 extend through the whole thickness of the layers, usually running 

 obliquely to the long axis of the cell, and often crossing one another 

 in the different thickening layers (Fig. 82). 



In a much-thickened cell wall, owing to chemical and optical 



