ROOTS. 413 



789. (5) Exposure of root surface for absorption. The prin- 

 cipal part of root absorption takes place in the young root and 

 the root hairs growing near the root-tip. The root-tips and 

 root-hairs in their relation to the root systems on which they are 

 borne are not to be compared morphologically with the leaves 

 and stern system. But the root-tips and hairs are absorbing 

 organs of the roots while the main root system supports them, 

 brings them into relation with the soil and moisture, and con- 

 ducts food and other substances to and from them. One of the 

 important relations of the leaf is that of light, and since the source 

 of light is restricted, i.e., it is not equally strong from all sides, 

 an expanded and thin leaf-blade is more effective than an equal 

 expenditure of plant material in the form of thread-like out- 

 growths. It is different, however, with the plant food dissolved 

 in the soil water. It is equally accessible on all sides. A greater 

 surface for absorption is exposed with the same expenditure of 

 material by multiplication of the organs and a reduction in their 

 size. Numerous delicate root-hairs present a greater absorbing 

 surface than if the same amount of material were massed into 

 leaflike expansions. There is another important advantage 

 also. Its slender roots and thread-like root-hairs allow greater 

 freedom of circulation of water, food solutions, and air than if 

 the absorbing organs of the roots were broadly expanded. 



790. (6) The renewal of the delicate structures for absorp- 

 tion. The delicate root-hairs are easily injured. The thin 

 cell-walls through which food solutions flow become more or less 

 choked by the gradual deposit of substances in solution in the 

 water, and continued growth of the root in diameter forms a 

 firmer epidermis and cortex through which the solutions taken 

 up by the root-hairs would pass with difficulty. For this reason 

 new root-hairs are constantly being formed on the growing root- 

 tip throughout the growing season, and in the case of perennial 

 plants, through each season of their growth. 



791. (7) Aid in preparation of food from raw materials. For 

 most plants the food obtained from the soil is already in solution in 

 the soil water. But there are certain substances (examples, some 



