METABOLISM 



273 



teristic shape (fig. 132, A, B). In these cases the cluster 

 of crystals is usually invested by a delicate skin derived 

 from the protoplasm, thus shutting it off completely from 

 any participation in the metabolism of the cell in which it 

 lies. 



Carbonate of calcium may also be deposited in the 

 substance of the cell-wall, or of protrusions from it, as 

 in the cystoliths of Ficus, Urtica, and other plants 

 (fig. 129). 



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 



FIG. 128. CRYSTALS OF OXALVTE OF 

 CALCIUM. A, FROM BEET (Sphcsra- 

 phides] ; B, FROM ARUM (Baphides). 



FIG. 129. SECTION OF PORTION 

 OF LEAF OF Ficus, SHOW- 

 ING CYSTOLITH (cys) IN 

 LARGE CELL OF THE THREE- 

 LAYERED EPIDERMIS (ep) 



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 

 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 



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