METABOLISM 273 
teristic shape (fig. 182, 4, 8). 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 (Hquisetwm). 
Though many of these substances, both excretions and 
bye-products, are of no value for nutrition, some of them 
Fig. 129,—Srction oF Portion 
oF Lear or Ficus, sHow- 
Fic, 123.—Crysraus oF OXaLitTE OF ING CySsTOLITH (cys) IN 
CaLcium. 4, From Bret (Sphera- LARGE CELL OF THE THREE- 
phides); 8, FRom ARuM (Raphides). 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 
18 
