94 BOTANY AND PHARMACOGNOSY. 



a group of thin-walled parenchyma cells, which constitute the 

 ROOT-CAP (Fig. 60). The root-cap varies in length, and it is 

 only in rare cases that the roots of land plants are without a cap. 

 In addition to its protective function the root-cap makes the 

 passage of the roots through the soil easier by reason of the 

 mucilaginous character of its outer cell-membrane. 



Just above the root-cap there is developed a narrow zone of 

 delicate hairs, which arise from the surface cells and are usually 

 thin-walled and unicellular. These are known as root-hairs 

 (Fig. 59, B) and their function is twofold: (i) They secrete an 

 acid which renders the inorganic substances of the earth soluble, 

 and (2) they absorb these and other substances for the nourish- 

 ment of the plant. It should be stated that there are a number of 

 plants which for various reasons do not possess root-hairs, such 

 as water-plants, marsh-plants, certain Coniferas, etc. 



When the germinating root persists, it is called the main, tap 

 or primary root (Fig. 88) ; when it divides into a number of 

 similar branches it is known as a multiple primary root, as 

 in dahlia. Branches may arise in addition on both kinds of roots 

 (Fig. 88). 



There are some plants which complete their cycle of growth 

 in one year, and such plants are spoken of as annuals. Other 

 plants produce only roots, stems and leaves the first year. The 

 roots are large and fleshy and contain large quantities of nutritive 

 matter, which is stored for the use of the plant the following 

 year, when it produces fruit and thus completes its cycle of 

 growth. Plants of this class are known as biennials, and the 

 roots as root-tubers or tuberous roots. Plants which persist year 

 after year are known as perennials. Sometimes, as in temperate 

 climates, the roots only are perennial, as in Phytolacca decandra, 

 when the plant is spoken of as a perennial herb. 



Influence of Gravity. — The root is popularly supposed to 

 grow downward, in order to avoid the light. The roots of plants 

 grown in hanging baskets, however, naturally emerge into the 

 light, and then continue their downward growth. On the other 

 hand, the theory has been established (as a result of Knight's 

 experiments) that the root grows downward by reason of the 

 influence of gravity. In addition it may be said that the principal 



