^8 CELLULAR AND VASCULAR SYSTEMS, [en. 



composition with the ternary substance of which the cell- 

 wall is formed, from which it differs in being a temporary 

 instead of a permanent formation. It originates always 

 within the chlorophyll-containing granules of the leaves, 

 whence it is conveyed, in what we may here conveniently 

 speak of as in solution, to those tissues in which growth 

 is actually taking place, or to others destined to receive 

 the reserves of food available for future use : in which 

 latter case it reassumes its characteristic form through the 

 agency of protoplasmic corpuscles destitute of green colour, 

 but in other respects corresponding to the chlorophyll- 

 granules in which the starch was first formed. Under high 

 magnifying power the individual grains present more or less 

 distinctly the appearance of successive zones around a 

 usually eccentric focus. 



Another form in which temporary reserves are stored 

 up in the cells, we find in the globules of oil abundant in 

 some cells, especially of certain seeds and fruits. Hence 

 we find the principal source of our vegetable oils in the 

 fruits of different plants, as Rape-seed (embryo), Olive 

 (pulp of drupe), Cocoa-nut (albumen), c. 



Sugar is another food-deposit of plants, differing from 

 starch in being soluble in the watery cell-sap which fills 

 the full-grown cells. It would appear to be usually an 

 intermediate condition of the plant's ternary food, either 

 preceding the formation or immediately following the 

 solution of starch. It abounds in the cell-sap of the 

 stem of the Sugar-cane and in the root of the Beet. 



9. Besides chlorophyll, oil, and starch, there may fre- 

 quently be found minute crystals, either in the form of 

 microscopic needle-like prisms (raphides), or collected into 

 nodules (sphczraphides\ lying in the cavity of cells or, 

 indeed, actually embedded in their walls. These are usually 

 salts of lime, and in themselves of subordinate importance. 



10. Besides the distinction already pointed out be- 

 tween cells and vessels, you may roughly group the 

 different kinds of cells as long or short. Long cells are 

 usually thick-sided and often taper at each end, so that 

 when a number of them are associated together, forming a 

 tissue, we find such tissue to be generally firm and tough. 

 Such cells, together with a few vessels, form the principal 

 mass of wood, of petioles, and of the "veins of leaves. 

 These veins, which have nothing in common with the 



