CHEMISTRY OF MANURES 171 



heart, for the veins and arteries of a leaf carry up and 

 down sap and nutriment, and can only do this in response 

 to the movement of the young buds which first begin to 

 respond to the magic of spring sunshine, when the sap 

 hurries to their aid and expands them into full-sized 

 leaves . Again , the surfaces of leaves are closely analogous 

 to the skin of animals, for they absorb or exude moisture 

 in varying degrees by opening and closing the stomata 

 spoken of according to the changes of humidity and 

 temperature. (See Plate 42.) 



Furthermore the leaves act as a stomach inasmuch as 

 they take in the nutriment sent up by the sap and accept 

 or reject the items on the '' menu " as the case may be, 

 the rejection, chiefly water (there does not appear to be a 

 wine list), being got rid of as mentioned above. 



How does the plant and its leaves perform all these 

 things ? It is of course largely a mystery, but turning 

 again to our leaf section on Plate 42 we see that the inner 

 cells are filled with some substance shown dotted. This 

 is chlorophyll or plant green, which has been mentioned 

 before, and' in it Ues the clue to our plant miracle, in all 

 probabiHty, for it has been proved that chlorophyll has a 

 strange power of selection and analysis over the gases and 

 liquids which come to the making of a vegetable structure. 

 The cells shown in white on the top of the leaf, forming 

 its covering, are simply filled with air and are transparent. 

 The green flesh of the leaf shines through, much as in the 

 case of an albino, like a white rabbit, where the eyes, 

 being devoid of pigment, appear to be pink on account 

 of the blood that shines through them. 



It is chlorophyll which enables a plant to feed upon 

 inorganic substances, but this material also co-operates 

 with the sun's rays and is in fact rather hke a storage 

 battery which absorbs the energy that pours down from 

 heaven in beams of light on a bright day, so that fermenta- 

 tion soon takes place all through the leafy structure and 

 down the '' winding stair " of the plant, which thus grows 



