HOW PLANTS OBTAIN WATER, 3 1 



and we must next turn our attention to the way in which these 

 solutions are carried farther into the plant. We should make a 

 section across the root of a seedling in the region of the root 

 hairs and examine it with the aid of a microscope. We here see 

 that the root hairs are formed by the elongation of certain of the 

 surface cells of the root. These cells elongate perpendicularly to 

 the root, and become $mm to 6mm long. They are flexuous or 

 irregular in outline and cylindrical, as shown in fig. 43. The 

 end of the hair next the root fits in between the adjacent superfi- 

 cial cells of the root and joins closely to the next deeper layer of 

 cells. In studying the section of the young root we see that the 

 root is made up of cells which lie closely side by side, each with 

 its wall, its protoplasm and cell-sap, the protoplasmic membrane 

 lying on the inside of each cell wall. 



61. In the absorption of the watery solutions of plant food by the root 

 hairs, the cell-sap, being a more concentrated solution, gains some of the 

 former, since the liquid of less concentration flows through the protoplasmic 

 membrane into the more concentrated cell-sap, increasing the bulk of the lat- 

 ter. This makes the root hairs turgid, and at the same time dilutes the cell- 

 sap so that the concentration is not so great. The cells of the root lying in- 

 side and close to the base of the root hairs have a cell-sap which is now more 

 concentrated than the diluted cell-sap of the hairs, and consequently gain 

 some of the fpod solutions from the latter, which tends to lessen the content 

 of the root hairs and also to increase the concentration of the cell-sap of the 

 same. This makes it possible for the root hairs to draw on the soil for more 

 of the food solutions, and thus, by a variation in the concentration of the sub- 

 stances in solution in the cell-sap of the different cells, the food solutions are 

 carried along until they reach the -vascular bundles, through which the solu- 

 tions are carried to distant parts of the plant. Some believe that there is a 

 rhythmic action of the elastic cell walls in these cells between the root hairs and 

 the vascular bundles. This occurs in such a way that, after the cell becomes 

 turgid, it contracts, thus reducing the size of the cell and forcing some of the 

 food solutions into the adjacent cells, when by absorption of more food solu- 

 tions, or water, the cell increases in turgidity again. This rhythmic action of 

 the cells, if it does take place, would act as a pump to force the solutions 

 along, and would form one of the causes of root pressure. 



62. How the root hairs get the watery solutions from the soil. If we 

 examine the root hairs of a number of seedlings which are growing in the soil 

 under normal conditions, we shall see that a large quantity of soil readily 

 clings to the roots. We should note also that unless the soil has been recently 

 watered there is no free water in it ; the soil is only moist. We are curious 



