NUTRITION. THE MACHINERY. 31 



and bits of leaves (Darwin). Other plants absorb ammonia 

 by means of the hairs covering their leaves, and this class 

 is probably more numerous than the foregoing. Others, 

 again, have no faculty of digesting by their leaves, though 

 they absorb solutions of decaying animal matter by their 

 means. Some, such as the bird's nest orchis, feed on the 

 decay of vegetable matter, and are themselves nearly or 

 quite destitute of chlorophyll. Lastly, there is the class 

 of true parasites, such as the broomrapes (Orobanche) and 

 dodders (Cuscuta), which affix themselves to living plants, 

 and being themselves destitute of chlorophyll, are unable 

 to live, except at the expense of the plants upon which 

 they grow. 



Transpiration of Water. There is a large absorption 

 of water, as has been said, by means of the root, and in 

 some cases, at any rate, there is an absorption of the same 

 fluid or vapour by means of the leaves. On the other 

 hand, there is a loss of water or watery vapour from the 

 surface, which is sometimes so profuse as to cause the 

 plant to wither and flag. "We have only to place some 

 leaves in a cool tumbler, and expose them to the light, to 

 see the condensed water on the sides of the glass. The 

 quantity of watery vapour emitted in sunlight by the green 

 surfaces of plants is enormous, and it has been shown 

 experimentally that it is the chlorophyll which is 

 largely concerned in this outflow, for where that substance 

 is deficient or wanting, transpiration of fluid is proportion- 

 ately reduced or stopped. But while bright light, such as 

 that furnished by the red and yellow ray of the solar 

 spectrum, is most efficacious in stimulating the decompo- 

 sition of carbonic acid, it is the blue ray which specially 



