134 



BOTANY 



PART I 



in length ceases, the ROOT-HAIRS ( 69 ) (Fig. 158 r, Fig. 51), which are 

 important appendages of subterranean roots, appear. They are 

 localised tubular protrusions of the living epidermal cells with thin 

 walls covered with mucilage. When seedlings, e.g., of Wheat are grown 

 in a moist chamber they can be seen with the naked eye, forming a 

 delicate down on the surface of the root. They occur in enormous 

 numbers (e.g. about 42T) per sq. mm, in Zea 

 Mays). Their length varies, according to the 

 kind of plant, between 0'15 and 8 mm. They 

 enlarge the surface of the root greatly (in 

 Pisum, for example, twelvefold) and penetrate 

 between the particles of the soil and become 

 attached to them. Thus in the soil they do 

 not retain the cylindrical form seen in moist 

 air but are bent to and fro, and flattened, 

 club-shaped, or lobed at the top (Fig. 239). 

 They serve to absorb water and dissolved salts. 

 They only live for some days, the older root- 

 hairs dying off as new ones form nearer the 

 tip : thus only a limited zone of the young 

 root some centimetres or millimetres in length 

 is clothed with them. The older smooth portion 

 of the root serves for conduction, but has 

 ceased to absorb the water. The surface often 

 shows transverse wrinkling brought about by 

 subsequent contraction of this region of the 

 root. This shortens the root so that, like a 

 tense support, it anchors the shoot more firmly 

 in the soil (cf. Fig. 207, 6). 



Root- hairs are wanting in some plants, especially 

 those which can readily obtain water, as is the case 

 with many aquatic and marsh plants. The roots of 

 Fio. 158. Seedling of Carpinns some aquatic plants, such as Nuphar luteum, form 

 Betulus. r, Zone of root- roo t-liairs when they penetrate the soil ; the roots of 

 hairs near root-tip; h, hypo- h , t . h C arex paludosa, when there is 



cotyl; hw, mam root'; sw, f 



lateral roots; I, V, leaf; e, lack ot water, 

 epicoty 1 ; c, cotyledons. (Nat. 



size. After NOLL.) 3. Primary Structure of the Root 



When the transformation of the meristematic 



cells into permanent tissue has taken place the same kinds of tissue 

 are recognisable in roots as in shoots, their arrangement being as a rule 

 radially symmetrical. 



The surface of the younger portions of the root is bounded by the 

 thin- walled EPIDERMIS which, with the root-hairs borne upon it, serves 

 for absorption. The ABSENCE OF STOMATA and of a CUTICLE is 

 characteristic of this layer. The epidermis of the root dies off with 



