METABOLISM 



279 



matters are often deposited in the substance of cell- walls. 

 The oxalate of calcium occurs frequently in this situation 

 (fig. 127). In other cases it is deposited in special cells, 

 where it forms clusters of crystals of characteristic shape 

 (fig. 128, A, B). In these cases the cluster of crystals is 

 usually invested by a delicate skin derived from the proto- 

 plasm, thus shutting it off completely from any participa- 

 tion in the metabolism of the cell in which it lies. 



Carbonate of calcium may also be deposited in the sub- 

 stance of the cell-wall, or of protrusions from it, as in the 

 cystoliths of Ficus, Urtica, and other plants (fig. 129). 



FIG. 128. CRYSTALS OF OXALATE OF 

 CALCIUM. A, FROM BEET (Sphcera- 

 phides) ; B, FROM ARUM (Raphides). 



ej 



FIG. 129. SECTION OF PORTION OF 

 LEAF OF Ficus, SHOWING CYSTO- 

 LITH (cys) IN LARGE CELL OF THE 

 THREE-LAYERED EPIDERMIS (ep). 

 (pa) PALISADE LAYER, 



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 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 



