278 DISEASE IN PLANTS. 



It is these formed products of metabolism (Meta- 

 bolites), especially cellulose and bodies which 

 result from its subsequent transformation, which 

 constitute the main permanent mass of the 

 ordinary plant. 



We are now in a position to see how another 

 fundamental cause of disease or predisposition to 

 disease exists in the deprivation of the protoplasm 

 of any of the elements needed to supply in the 

 food-materials the place of those which have 

 been permanently put aside in the form of cell- 

 walls, or burnt off in respiration, passed out as 

 excretions, or in other ways lost. 



It is clear that the indispensability of an element 

 must mean that the protoplasmic molecule cannot 

 be completed without it : the same conclusion is 

 supported by the experimental proof that these 

 elements cannot be replaced by chemically similar 

 elements. 



It does not follow, however, that the protoplasm 

 molecule must always have the same number of 

 atoms of these elements, and grouped always in 

 the same atom-complexes before being assimilated ; 

 nor that the protoplasm molecule, when once built 

 up, always breaks down in exactly the same waw 

 On the contrary, while the protoplasm of corre- 

 sponding parts of a daisy and of a rose must 

 contain all the elements named, we must believe 

 that the atom groupings are different in the 

 protoplasm molecule in each case ; and though 

 the molecules of the cell-protoplasm, of the 

 nucleus, of the chlorophyll-corpuscles, etc., of one 



