MORPHOLOGY OF HIGHER PLAXTS. 301 



Upon examining the tip of a very young root by means of the 

 microscope, it will be seen that the growing point is protected by 

 a cup-shaped body of a more or less solid structure and frequently 

 mucilaginous; this is known as a root-cap. Its function is to 

 protect the growing point, and it exists in all roots of terrestrial, 

 epiphytic, and aquatic plants except the parasites. 



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-iiairs 

 (Fig. 161, 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 Coniferae, Ericaceae, etc. 



When the primary root persists (as in Gymnosperms and 

 Dicotyledons) it increases considerably in length and becomes 

 ramified ; if, at the same time, it increases in thickness, and much 

 more so than its branches, then it is called a tap-root (as in 

 carrot, beet, etc.). 



In the vascular cryptogams (Pteridophytes) and the monocoty- 

 ledons the primary root is generally thin and weak, frequently 

 but little ramified, and disappears at an early stage, being re- 

 placed by SECONDARY ROOTS^ as in Zea. Secondary roots may 

 arise not only upon the stem but even upon leaves, as in Begonia 

 and Bryophyllum. The term lateral roots is restricted to those 

 that develop from the root alone. 



The development of roots upon shoots or of so-called '* ad- 

 ventitious roots'" occurs in nearly all of the woody plants of 

 the Spermophyta. Many annual herbaceous plants do not possess 

 this capacity at all. The adventitious roots arise from " root- 

 primordia " which are formed under the cortex of the shoots. 

 While ordinarily they do not develop upon the shoots, yet if 

 cuttings are made, as of Coleus, Geranium, Rosa, etc., we find 

 " either singly or on both sides of the axillary buds " the develop- 

 ment of adventitious roots from the latent root-primordia. 



Influence of Gravity. — The root is popularly supposed to 

 grow downward, in order to avoid the light. On the other hand, 



