PLANT PHTSIOLOOT. in 



experiments the rate of diffusion varies greatly, and is de- 

 pendent upon (a) the solution itself, {h) the substance in 

 which it diffuses, and (c) the temperature; thus hydro- 

 chloric acid diffuses more than twice as rapidly as common 

 salt, and seven times as rapidly as cane-sugar. This law 

 mus^j hold for solutions in plants also. 



134. Absorption of Gases. — Gases, also, are absorbed 

 directly by living cells, and these are diffused through other 

 gases in the plant, or they enter into watery solutions, as 

 described above. 



135. Assimilation, — In all the foregoing the plant is 

 simply taking material, but the latter does not yet properly 

 constitute a part of its living substance. It is still plant- 

 food, and must undergo certain important chemical changes 

 before it becomes a part of the plant itself. These chemical 

 changes in the aggregate constitute Assimilation. 



136. Carbon-assimilation. — The best-known assimilative 

 processes are those by which the plant obtains its carbon, 

 hence called carbon-assimilation. The first of these proc- 

 esses ( -pliotosyntax or pliotosyntliesis) results in the forma- 

 tion of a carbohydrate, commonly starch (CjHijOJ from 

 carbon dioxide (COJ and water (H^O), and to this the term 

 assimilation has until recently been restricted. "When u cell 

 containing chloroplasts absorbs carbon dioxide, the latter 

 unites with the water and forms carbonic acid (IIjCOj), 

 which is much more easily decomposed than eitlier the car- 

 bon dioxide or the water. In sunlight (or any similar light 

 of sufficient intensity) this carbonic acid is broken up by 

 the protoplasm of the chloroplasts, and a new compound 

 (probably formic aldehyde, CH^O) is formed, while at the 

 same time the excess of free oxygen (OJ is given off; Now 

 six molecules of CH^O equal CJI,,0„ glucose or grape- 



