14 INTRODUCTORY [chap. 



further 2 per cent, or so. These ash constituents com- 

 prise sulphur, phosphorus, silicon, and chlorine, among 

 the non-metals ; potassium, sodium, calcium, magnesium, 

 and a little iron and manganese, among the metals. 

 Traces of other metals occur from time to time in the 

 ashes of plants growing on soils which happen to 

 contain them, but they are unessential and may in this 

 connection be neglected. 



Carbon, then, is the main element in the plant's 

 economy, and we know that it is obtained by the plant 

 from the carbonic acid in the atmosphere through the 

 agency of the living cells in the leaf, which contain 

 green chlorophyll. The carbonic acid is taken in 

 through the small openings in the skin of the leaf, the 

 stomata ; it is decomposed by the chlorophyll-containing 

 cells, and the carbon is retained in combination with 

 the elements of water, so that it is first identifiable as 

 sugar and then as starch ; at the same time oxygen is 

 returned to the atmosphere. This decomposition is 

 one that necessitates an external supply of energy, 

 which is found to be derived from the light incident 

 upon the leaf, the process stopping in darkness, and 

 for low illuminations becoming proportional to the 

 amount of light falling upon the leaf. 



The conditions affecting this process of photo- 

 synthesis — the fundamental reaction of the whole plant- 

 world — have been subjected to considerable examina- 

 tion of late. In the method adopted by H. T. Brown, 

 the leaf, which may still be attached to the plant, is 

 enclosed in a flat air-tight box with glass sides, through 

 which sweeps a rapid but measured current of air. The 

 issuing air which has passed over the leaf is led into 

 an apparatus for the determination of the carbonic 

 acid (and, if need be, of the water) it contains ; at the 

 same time a parallel experiment without the leaf 



