ABSORPTION BY THE ROOTS. 89 



destroyed by an electric shock,) there is no room for any addi- 

 tional fluid within the system, and the absorption is checked also. 



121. When the upper part of the stem is cut off, the sap will 

 continue to rise by the force of Endosmose in the roots, so long 

 as the fluid within is of greater density than the fluid without. 

 But that will soon cease to be the case, the actions of the leaves 

 being destroyed, and no descending sap being intermixed to 

 keep up the force. These two causes, then, the absence of any 

 demand for sap in the leaves, and the cessation of the condition 

 necessary for the maintenance of Endosmose, are quite sufficient 

 to account for its absence in the dead plant ; and its performance 

 soon becomes impossible for another reason, the decay of the 

 soft tissue of the spongiole, through which it is performed. 



122. The nature of the fluid absorbed by the roots of plants 

 will be more fully discussed in Chapter vi. ; when their food, 

 and the mode in which they are nourished by it, will be described. 

 It may, however, be stated here, that they appear to have a 

 certain power of selection, some of the substances dissolved in 

 the fluid which surrounds the roots being absorbed, and others 

 being rejected. Thus, if a grain of Wheat, and a Pea, be grown 

 in the same soil, the former will obtain for itself all the silex or 

 flinty matter, which the water can dissolve ; and it is the depo- 

 sition of this in the stem, which gives to all the Grasses so much 

 firmness.* On the other hand the Pea will reject this, and will 

 take up whatever calcareous substances (or those formed of lime 

 and its compounds) the water of the soil contains, these being 

 rejected by the Wheat. Again, if the roots be placed in water, 

 coloured by any substance, of which the particles are very minute, 

 the finest of these will be absorbed with the fluid, and will be 

 carried to the leaves ; whilst the coarser ones are left behind. In 

 the same manner, if the roots be immersed in a solution of gum 

 or sugar, a certain proportion only of these substances will be 

 taken up with the fluid in which they are dissolved ; and that 

 which remains will thus become gradually thicker. 



* There is enough silex in a Wheat-straw to make a bead of glass, when melted 

 with potash wit!) the blow-pipe ; and in the Bamboo it is sometimes collected in the 

 knots in large masses, forming the substance called tabasheer in the East. See 

 also . 201. 



