CHAP, ii.] CHEMISTRY, ETC., OF PLANT LIFE. 61 



must be remembered that in the case of forests all the 

 inorganic materials removed from the soil are being 

 constantly returned in the form of dead leaves and 

 wood, so that practically the supply remains the same in 

 quantity, and is used up over and over again by different 

 generations of plants or even by the same plant. The 

 object of manure is to artificially replace plant food in 

 the soil when crops are grown and the produce removed. 



The influence of light on plant life is exercised in a 

 variety of ways, the following being amongst the most 

 important : 



Assimilation. This term is generally used to express 

 the decomposition of carbonic dioxide and water by 

 chlorophyll under the influence of light, and thus differs 

 considerably from the function expressed by the same 

 term by the animal physiologist. As already explained, 

 assimilation takes place only in those superficial cells 

 containing chlorophyll, and further the chlorophyll can 

 only exercise its function when exposed to light. The 

 superficial cells of most young parts of plants growing 

 in the air contain chlorophyll, and consequently assist in 

 assimilation; but leaves are the most important organs 

 in this connection, their general structure and flattening 

 out into a thin sheet being for the purpose of exposing 

 the greatest amount of surface from a given amount 

 of material. Other important functions performed 

 by leaves, as respiration, transpiration, etc., will be 

 explained at a later stage. 



The general structure of a typical leaf growing in a 

 more or less horizontal direction is as follows. Every 

 part is covered with the epidermis, and as a general 

 rule the stomata or openings through the epidermis are 



