56 



VEGETABLE PHYSIOLOGY 



cell-\vall. The chief of these are salts of calcium, usually 

 the oxalate, but often the carbonate. Some cell-walls show 

 a copious deposit of regular crystals of one of these such 

 are the cells of the bulb scales of the onion, the fibres of 

 the bast of Ephedra, among others (fig. 48). In many plants 

 copious deposits of silica are formed in the cell-wall, 

 especially in the epidermal cells of the Equisetacece, and in 

 those of the cereal grasses. The value of this deposit to 

 the plant is not very evident ; it appears at first sight to be 

 an adaptation enabling the plant to remain upright, but it 



FIG. 48. CRYSTALS or CALCIUM 

 OXALATE IN WALL OF CELL 

 OF THB BAST OF Ephedra. 



FIG. 49. SECTION OF PORTION 

 OF LEAF OF Ficus, SHOW- 

 ING CYSTOLITH (cys) IN LARGE 

 CELL OF THE THREE-LAYERED 

 EPIDERMIS (ep). (pa) PALI- 

 SADE LAYER. 



is found that its absence does not render the grasses more 

 liable to fall. 



Some cells of the epidermis of certain plants, especially 

 among the Nettle family, contain curious ingrowths of 

 cellulose, in which there is a very large deposition of 

 calcium carbonate. They are known as cystoliihs (fig. 49). 



The cell-walls of certain regions of particular plants 

 are transformed into mucilage. This material is especially 

 prominent in the large brown seaweeds, particularly the 

 Fucacece, where it forms the bulk of the internal tissue. It 

 occurs also in certain layers of the seed-coats of such seeds as 

 linseed, and in certain regions in the sporocarps of Marsilea. 



