ii8 



The Living Plant 



stances which give the pleasant acid taste to fruits. Thus, mahc 

 acid gives the tart taste to apples and currants, citric acid to 

 lemons and oranges, tartaric acid (from which cream of tartar 



is made) to grapes. In all of these 

 cases there is a reason, as our 

 chapter on Dissemination will 

 show, why these fruits should be 

 eaten by animals, to which the 

 acids certainly serve to render the 

 fruits more attractive. On the 

 other hand tannic acid, which oc- 

 curs in the bark of many plants, 

 (and from which man extracts it 



Fig. 37.-A gland, highly magnified, ^^Y tanning leather), haS an as- 

 formed by a fusion of several cells tringent taste unpleasant to ani- 



containmg a large drop of an ethereal ^ ^ 



oil, as seen in a cross-section of a leaf mals, against whicll, aCCOrdiugly, 



of Dictamnus Fraxinella. 



its presence has some tendency to 

 protect the plant tissues. These acids, which all occur in solu- 

 tion in the sap, have a comparatively simple composition, the 

 formula of malic acid, for example, being C4Hg05. Their mode 

 of formation is not entirely understood. 



Plant Waxes. These occur chiefly on the surface of plants, 

 where they constitute the bloom, commonly of bluish color, wliich 

 is familiar upon plums and some leaves, On the dry berries of 

 the Bayberry, a common plant of the coast, a wax accumulates 

 in such quantity that in early days it was gathered and used for 

 the making of candles. In general the waxes seem to render 

 plants immune against wetting, after the manner of the oil on 

 the back of the proverbial duck, — the disadvantage of the wet- 

 ting being this, that the water would clog the stomata, and hence 

 prevent the passage of gases that are needed in photosynthesis. 

 If, now, the reader should ask me why, when the wax is thus of 

 advantage, so many plants do not have it, I would answer by 

 asking in turn why it is, that, if riches are such an advantage (or 



