Ch. Ill, 13] USES OF THE PLANT'S FOOD 



111 



where their bitter, astringent taste has been supposed to 

 protect the trees against rodents and insects, while a certain 

 antiseptic quality prevents development of parasitic Fungi 

 and hence decay of the bark. It is the oxidation changes 

 in these tannins under weathering which give the dark brown 

 color to old bark. Having incidentally the remarkable prop- 

 erty of hardening the gelatine in skins, they are utilized 

 by man for tanning leather, though here again the chemist is 

 providing artificial substitutes. The plant waxes occur as 

 the "bloom" upon some fruits and leaves, and at times, as 

 in the Bayberry of the coast, such a wax is abundant 

 enough to be collected and used for candles, as our forefathers 

 found; but the meaning of the wax to the plants is not 

 certain. And other secretions occur, of more special kind 

 and mostly uncertain significance. 



Rather common in plants are crystals, frequently, though 

 not always, in cells differing from their neighbors; and 

 they often exhibit marked 

 beauty of form (Fig. 68). 

 They are composed chiefly 

 of oxalate or carbonate of 

 lime, and represent not 

 secretions but excretions; 

 for they seem to be either 

 useless by-products of func- 

 tional chemical reactions, 

 or else substances brought 

 into the plant from the soil 

 with the water, and not 

 needed in growth. The 

 plant has no continuously-acting excretion system such as 

 the higher animals possess, but instead accumulates waste 

 matters in out-of-the-way cells, often in leaves and bark, the 

 fall of which does incidentally provide an excreting system. 



5. RESPIRATION. The photosynthetic sugar has one 

 other use, not at all inferior in importance to any yet 



Fig. 68. — Crystals of calcic oxalate, 

 in a cell of Begonia ; much magnified. 

 (After Kny.) 



