THE STEUCTUEE OF THE ROOT 19 



Of these three zones of the root, the median one, in which 

 the absorption of salts takes place, is undoubtedly the most 

 important for the general economy of the plant. It is repre- 

 sented in Fig. I as covered with root-hairs. "We must not, 

 however, forget that the production of root-hairs is not 

 essential for absorption, but that the tabular cells of the 

 smooth epidermis are quite capable of absorbing the necessary 

 solutions. But in that case the quantity absorbed by each 

 cell will naturally be very much smaller than that absorbed 

 by a cell which has elongated to form a root-hair, and may 

 increase its absorptive surface a hundredfold. 



Every cell of the absorbing epidermis of the root represents 

 a closed space, and yet, as soon as the plant is in want of it, 

 water with its dissolved salts enters through the cell- wall into 

 the closed cell space. This method of passage of gaseous or 

 liquid substances through an uninjured membrane is termed 

 diffusion, and it can be explained by the assumption that the 

 cell-wall, just like all other organic structures, is composed of 

 very small particles (micellm), between which there remain 

 very minute interstices, so minute as not to be visible with 

 our strongest optical instruments. The presence of these 

 interstices makes every membrane into a very delicate filter, 

 and allows the passage of all bodies which are small enough 

 to pass through them, and which are attracted by the cell- 

 contents. 



The attractive centre of each cell is the substance we have 

 termed protoplasm. The richer a cell is in protoplasm, the 

 greater will be its need for nourishment, and the stronger its 

 power of attraction. As the inner cells of the. cortex are 

 richer in protoplasm than the outer ones, they will be stronger 

 centres of attraction, and consequently a current will be set 

 up in the direction of the central cylinder. The vessels of 

 the latter are always completely or partially filled with water, 

 and form a continuous, though sometimes curiously contorted, 

 system of pipes, leading to the leaves, and there they give off 

 to the green cells as much water and salts as the latter require. 

 The leaves lose daily large quantities of water by transpiration, 

 and the vessels which have to supply the leaves must make 

 up the deficiency by absorbing water from below. 



