38 PLANTS AND MAN 



oxygen which is a by-product of this process, if not completely 

 used up within the plant by respiration, passes out of the cells 

 and into the atmosphere via the stomates. A more detailed 

 description of food manufacture is taken up in Chapter 6. 



From this we can see the importance of leaves to a plant. In 

 undifferentiated plants where chloroplasts occur in every cell and 

 where there is little dependence of one part of the plant body 

 upon another, the loss of a portion of the body is of little conse- 

 quence. But the whole life of the maple tree depends upon the 

 activity ot the leaves; every cell lacking chloroplasts — which 

 includes most of the roots, stems and branches, flowers, fruits 

 and seeds — is dependent upon the food-manufacturing leaf cells. 

 Therefore the loss of leaves, due to insect defoliation or fungus 

 attack, becomes a serious matter. The dependence also works the 

 other way. The leaves must rely upon stem growth to obtain 

 sufficient light and upon co-operation of root and stem to bring 

 to them the raw materials upon which the life of the leaves them- 

 selves depend. The leaves are helpless and can not themselves 

 stay alive, it deprived of roots and stems. 



The intake of water and dissolved salts from the environment 

 occurs through other highly specialized portions of the maple 

 tree. The roots are subterranean organs with a structure suited 

 for their dual role of absorption and anchorage. The actual tip of 

 each ultimate branch of the root system consists of actively grow- 

 ing cells which bring about continuous growth and elongation of 

 the root; this takes place, however, in a far different medium 

 than that surrounding the growing tip of the stems. The root tip 

 has to force its way through the ground, past and around soil 

 particles with contours so harsh as to easily injure living cells. 

 To obviate this, the growing cells of the tip produce a buffer 

 root-cap of cells which are continually replaced as they wear 

 away. Behind the root-cap and the root tip of growing cells, the 

 cells become differentiated into three regions (fig. 17); a single 

 layered epidermis, a cortex of irregular and large parenchyma 

 cells, and a central cylinder of conductive and supporting cells. 

 In none of these regions of the root are there chloroplast-bearing 

 cells. The outer epidermis, unlike that of the leaf, is not special- 

 ized as an impervious protective coat, for through it all materials 



