xvi.] THE BEAN PLANT. 467 



The physiological processes which go on in the higher 

 green plants, such as the Fern and the Bean, resemble, 

 in the gross, those which take place in Protococcus and 

 Cham. For such plants grow and flourish if their roots 

 are immersed in water containing a due proportion of 

 certain saline matters, while their stem and leaves are 

 exposed to the air, and receive the influence of the sun's 

 rays. 



A Bean-plant, for instance, may be grown, if supplied 

 through its roots with a dilute watery solution of potassium 

 and calcium nitrate, potassium phosphate and iron sulphate, 

 and magnesium sulphate. While growing it absorbs the 

 solution, the greater part of the water of which evaporates 

 from the extensive surface of the plant. In sunshine, it 

 rapidly decomposes carbonic anhydride, fixing the carbon, 

 and setting free the oxygen ; at night, it slowly absorbs 

 oxygen, and gives off carbonic acid ; and it manufactures a 

 large quantity of protein compounds, cellulose, starch, sugar 

 and the like, from the raw materials supplied to it. 



It is further clear that, as the decomposition of carbonic 

 anhydride can take place only under the combined in- 

 fluences of chlorophyll and sunlight, that operation must 

 be confined, in all ordinary plants, to the tissue imme- 

 diately beneath the epidermis in the stem, and to the 

 leaves. And it can be proved, experimentally, that fresh 

 green leaves possess this power to a remarkable extent. 

 The decomposition of carbonic anhydride and of water 

 appears to go on simultaneously, and as the result of the 

 process, various carbohydrates, such as grape-sugar, make 

 their appearance. 



On the other hand, it is clear that, when a plant is grown 

 under the conditions described, the nitrogenous and mineral 

 constituents of its food can reach the leaves only by passing 



30 2 



