THE SKIN, OB BIND, OF PLANTS. 77 



wood, like the skin on the hands of a delicate lady, 

 remains thin and smooth. 



As plants increase in thickness, the rind is stretched 

 out to make room for the addition, as in the apple tree; 

 and in some fruits, such as the giant pear and the 

 pumpkin, this stretching out is considerable. In other 

 cases, such as on the trunks of the oak and elm, the 

 rind not being sufficiently stretchable, cracks into many 

 pieces ; while in other cases still, such as on the trunks 

 of the currant, the sycamore, and the birch, the rind 

 peels off every year, and a fresh one is formed, as is 

 the case with the outer skin in serpents. 



On flowers, leaves, and young branches, where the 

 rind is thin, it is usually transparent and colourless, 

 the colour which is seen being that of the inner bark 

 shining through it, as may be observed on the leaves 

 of oak, rose, bramble, holly, or sorrel, mined out under 

 the rind by minute caterpillars l ; but when it is thick 

 on the roots of trees it is usually coloured brown or 

 grey. 



Many various opinions have been held respecting the 

 origin and structure of the rind. 



Malpighi held that it is not a distinct membrane, 

 but is continuous with the bark. 



Hill and Mirbel, adopting a somewhat similar 

 opinion, think it is formed from the outer sides of the 

 cells composing the bark beneath it; but they are 

 obliged to suppose many plants, where this is not 

 observable, to be destitute of rind. 



(1) See INSECT ARCHITECTURE, p. 233. 



