THE ROOT AND ITS FUNCTIONS 55 



Other Osmotic Phenomena in the Plant. — Not only the 

 absorption of water from the soil, but the whole process of circula- 

 tion within the plant body, as well, is primarily an osmotic one; 

 for the salts taken in by the root-hairs, and any dissolved sub- 

 stances in other cells throughout the plant, move from cell to 

 cell by diffusion through the cytoplasmic membranes. 



Still another contribution of osmosis to the plant's activities 

 lies in maintaining the turgidity of the tissues. It is evident 

 that if a cell has a strong sap-solution and is thus able to absorb 

 water vigorously, it will become plump and fully expanded and will 

 press tightly against its neighbors. If all the cells become 

 turgid in this way the whole plant will tend to be erect and rigid, 

 like an inflated balloon. In the case of parts which do not possess 

 a firm skeleton, such as the leaf blades, floral organs, or other 

 comparatively soft structures, this turgidity is necessary to main- 

 tain their form, firmness, and proper functioning. Conversely, 

 if a cell is exposed to a solution of greater concentration than 

 its cell-sap, water will be withdrawn from it, it will collapse, 

 and its cytoplasm will be pulled away from the walls. Such a 

 condition of plasmolysis, if extreme or long-continued, will result 

 in the death of the cell; and, if extensive, in the death of the plant. 



Osmosis also plays an essential part in growth, for at any 

 growing region we find a point where the cells are multiplying in 

 number but are still small, and another point behind this where 

 each expands rapidly to its final size. This expansion, with the 

 consequent stretching of the cell-walls and growth of the tissues, 

 is due to the rapid absorption of water by the young and delicate 

 cells, the sap of which is rich in dissolved sugar. The force 

 exerted by any growing region is thus primarily due to osmotic 

 pressure. 



Other Functions of the Root. — We have briefly discussed the 

 root as an organ of anchorage and of storage, and in more detail 

 as an organ of absorption. It has less frequently certain other 

 functions which should be mentioned here. Roots may arise 

 from almost any part of the stem and sometimes from the leaves. 

 In many climbing plants they are produced abundantly on these 

 aerial organs and serve to hold the plant firmly to its support. 

 In corn, stout roots arise from the stem at a little distance above 

 the ground and pass into the soil, thus acting as props or guy- 

 ropes for the tall plant. In epiphytes, the roots are sent out 

 directly into the air and possess a characteristic spongy envelope 



