METABOLISM 



287 



Silica again is accumulated in the epidermis of many 

 grasses, and of the horsetails (Equisetum). 



Though many of these substances, both excretions and 

 bye-products, are of no value for nutrition, some of them 

 may play a very important part in the defence of plants 

 against their natural enemies, their nauseous smell or 

 flavour preventing their being eaten by animals, &c. Some 

 odours and the nectar found in flowers are doubtless of 

 great service in attracting insects, which assist in the 



FIG. 132. CBYSTALS OF OXALATE OF 

 CALCIUM. A, FROM BEET (Sphcera- 

 phides); B, FROM ABUM (Eaphides). 



FIG. 133. SECTION OF PORTION 

 OF LEAF OF Ficus, SHOW- 

 ING CYSTOLJTH (cys) IN 

 LARGE CELL OF THE THREE- 

 LAYERED EPIDERMIS (ep). 



process of cross-pollination, to be discussed in a subsequent 

 chapter. 



Though we cannot trace the formation of all these 

 various substances, both bye-products and excretions, 

 directly to the self-decomposition of the protoplasm, but 

 must regard them as formed partly by the processes of 

 oxidation and reduction, which we have seen are often 

 associated with its activity, and partly by subsequent 

 further decompositions of bodies originally thus formed, 

 we can trace them ultimately to protoplasmic activity, and 

 may consequently regard their formation as belonging to 

 the katabolic processes going on in the organism. 



