32 DISEASE IN PLANTS. 



accompanying respiration : if the seedlings are 

 allowed free access to light and carbon-dioxide^ 

 however, the amides disappear. The explanation 

 is that they are combined with some of the 

 materials of the carbohydrates, and again built up 

 into the material of the living protoplasm. 



Returning to our hypothetical starch-grain or^ 

 rather, its parts we have some of it retained as 

 starch, in excess, simply because it is not needed 

 at the moment : another portion gives up its 

 energy in respiration, and this does work on the 

 spot, or is lost as heat ; or in the body of an 

 organic acid, or its salt, the part in question may 

 do lifting or pressing work by osmosis, or cause 

 diffusion-currents from one cell to another. In 

 the constitution of the cell-wall we may have part 

 of our starch-grain aiding in imbibition or in the 

 establishment of elastic tensions in turgidity : and, 

 finally, parts may be built up into the living 

 protoplasmic machinery of the plant. 



What is true for the starch-grain is also true 

 for any particle of salt, or water, or gas which 

 enters into the metabolism of the living plant,, 

 regard being paid to the particular case, and 

 circumstances in each case. 



Enough has been said to show that the plant 

 cannot be properly studied merely as the subject 

 of chemical analysis or of physical investigation ; 

 you might as well expect to understand a watch 

 by assays of the gold, silver, steel and diamonds 

 of which its parts are made up, or to learn what 

 can be got out of the proper working of a lace 



