112 Human Physiology. 



Plants perspire like men, but seventeen times as much, and 

 only in the light or a dry air in motion. They purify the 

 atmosphere by absorbing carbonic acid which animals give 

 out, and also enrich it with oxygen. 



Every tree, by means of the spongioles at the extremities of 

 its rootlets, selects from the earth the nutriment it requires. 

 Wheat does not require the same as cabbage or clover, nor 

 the peach the same as the fir tree. The leaves and flowers of 

 plants also elaborate an extraordinary variety of substances 

 sugar, acids, honey, caoutchouc, gum, turpentine, the most 

 delicate flavours, the sweetest perfumes, the most nauseous 

 drugs, the most virulent poisons out of the same air, earth, 

 and sunshine. This can only be done by organs adapted to 

 such a wonderful chemistry, performing their functions by the 

 influence of an intelligence guiding the forces of life. The 

 rose and the poppy, the strawberry and the deadly hemlock, 

 all grow in the same garden bed. 



The growth of plants is sometimes wonderfully rapid. The 

 leaf of the turnip increases at times fifteen times its weight 

 in a minute the root has grown 15,990 times its original 

 weight in a day. And what a wonderful chemistry is involved 

 in all vegetable growth. A plant can decompose water and 

 carbonic acid; it takes hydrogen from the one, setting free its 

 oxygen ; it frees the oxygen and fixes the carbon of the other. 



There can be little doubt that the circulation of the sap or 

 juices in plants is as vital as that of the blood in animals. 

 Neither can be explained by chemistry or mechanics. But the 

 vitality of plants, and the action of what we must call intelli- 

 gence, is shown most strikingly in the wonderful organs and 

 processes of reproduction. In one day a whole tree is covered 

 with blossoms; the world is filled with the beauty and perfume 

 of flowers. Vital heat, a living property in all plants, rises in 

 some cases to the extent of fifteen degrees, so that the flower 

 is sensibly warm to the touch. The germ cells are formed i 



