HOW PLANTS OBTAIN THEIR LIQUID FOOD. 37 



If we recur now to the experiments which were performed 

 with the salt solution on the cells of spirogyra, in the cells of 

 the beet, and the way in which these cells become turgid again 

 when the salt solution is removed and they are again bathed 

 with water, we will have an indication of the way in which 

 plants take up nutrient solutions of food material through their 

 roots. 



It should be understood that food substances in solution 

 during absorption diffuse through the protoplasmic membrane 

 independently of each other and also independently of the rate 

 of movement^of the water from the soil into the root hairs and 

 cells of the roots. When the cell-sap is poor in certain sub- 

 stances which are dissolved in the surrounding water of the soil, 

 these substances diffuse inwardly more rapidly. But as the 

 cell-sap becomes richer in that particular food substance its 

 further absorption is correspondingly diminished until the cell- 

 sap becomes poorer again, as by diffusion this substance passes 

 on into other cells. 



74. How food solutions are carried into the plant. We can 

 see how the root hairs are able to take up solutions of plant 

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

 these solutions are carried further 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. 24. The end of the hair next the root fits in 

 between the adjacent superficial 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. 



