THE WHEAT CROP. 39 



ing plants, and these can only assimilate them when in a 

 soluble state. Without now venturing upon a discussion 

 of the important question of plant-nutrition, as to whether 

 the excretory theory of Decandolle, recently revived and 

 supported by Gasparini, 1 or the simpler mineral theory 

 of the chemists, is the soundest, we may recollect that the 

 power of the roots to absorb from the soil the various sub- 

 stances necessary for the plant is more than a mere 

 mechanical one, as, whether or not they have the power of 

 preparation, they unquestionably have the power of selec- 

 tion, and only select such as are necessary for their pur- 

 pose, and in a suitable state. They do not absorb indis- 

 criminately all matters they find in the soil in a soluble 

 state of which the inorganic are, of course, in excess 

 but appear to have the power of selecting those that are 

 desirable, and of refusing those which are not necessary 

 for their purpose. This power appears to be more de- 

 veloped in some plants than in others ; it exists, however, 

 in all, and is controlled, probably, by some difference in 

 the structure and substance of the pores or cells through 

 which the food passes into the extremities of the roots, 

 according to the different orders, or even genera of plants, 

 which exerts an influence upon their general powers of 

 absorption and assimilation. After the substances have 

 been absorbed by the roots, a chemical power or action is 

 called into play, and a change appears to take place in the 

 matter absorbed (food), as it is carried up by the ascending 

 juices (sap) of the plant towards the stem. Of these 

 changes, and the mode in which they are carried on, we 

 know but very little at present ; we only know that they 

 do exist, from the changed character of the substances 

 found in the sap. 2 



1 Ricerche sulla natura del succiatori e la escrezione delle radici, di Gul. 

 Gasparini, Napoli, of which an excellent digest was given in the Gardeners' 

 Chronicle, 1858, p. 19. 



2 The structure and functions of roots are fully described in Professor Hen- 



