136 PROTECTION OF THE GREEN TISSUE 



uninjured in field and wood, they have no odor 

 perceptible to man; but animals must recognize 

 these plants by the sense of smell, before they 

 have bitten and injured them. 



That the green leaves of the Rhododendrons 

 and Azaleas, of the Partridge-Berry, the Bearberry, 

 and of many other low evergreen plants, which 

 form a large part of the vegetation of pastures 

 and moors, as well as of the high mountain slopes, 

 are avoided by grazing animals is explained by the 

 fact that they are indigestible, owing to their tough 

 skin, often containing silica. It is certain that the 

 formation of a thick arid hard cuticle, and the pres- 

 ence of silica in the epidermis is a means of protec- 

 tion against grazing animals, by which, of course, 

 we do not mean to say that this structure has no 

 other function. 



Water is another excellent means of defence for 

 many plants. The water which is collected by 

 certain leaves from the rain and dew often remains 

 in special receptacles for days and weeks. Rumi- 

 nating animals do not graze in the morning when 

 the grass is covered with dew, they wait till the 

 cold drops are dried up, and even later in the 

 day they avoid those plants on which drops are 

 hanging. 



