180 



HUTCHINSON'S POPULAR BOTANY 



rather thickened cells (endoderm) surrounding the stele is parenchyma (pm), 

 which forms a strong padding and hermetically closes the central cylinder, 

 thus preventing the passage of air while allowing that of water. It is known 

 as the root-sheath. In most plants with biennial and perennial roots the root- 

 sheath serves the further purpose of a repository for food material starch, 

 fat, sugar, or whatever other supplies may be needed for the next period of 

 vegetation. Surrounding the tissue is a mass of cells (the cortex) consisting 

 of thinner-walled parenchyma, in which also reserve materials are deposited ; 

 and then, last of all, we have the epidermis (e), with its unicellular root- 

 hairs (/), those delicate organs by which the plant dissolves and through 

 which are absorbed the inorganic substances which constitute, with water, 

 the crude ascending sap. 



As a protection against field-mice, insect larvae, and other underground 

 animals, many food-storing roots develop poisonous and disagreeable sub- 

 stances in their tissues, in the way of noxious alkaloids, fretid gum resins, 

 and other products well known to druggists ; and it has been observed that 

 such roots are very seldom attacked. Protected roots of this kind will be 

 found in Soapwort, several species of Gentian (Gentiana punctata, lutea, and 

 pannonica), as well as of the thick and poisonous main-roots of Monkshood 

 (Aconitum napellus}, the massive roots of the Rhubarb (Rheum officinale}, and 

 many Umbeliiferce. 



The fact that the root is often a storehouse of nutritious food substances 

 has an important morphological bearing, almost all departures from a slender 



tapering form at least in the young root 

 being chiefly due to it. The Carrot and 

 Turnip, for example, are simply the primary 

 roots of Daucus carota and Brassica rapa 

 swollen up with reserve material (figs. 

 228-233; see p. 183). These primary or 

 main roots are known as tap-roots ; though 

 various qualifying names such as conical, 

 fusiform, or spindle-shaped, and napiform 

 or turnip-shapedare given, according to 

 the special form which the tap-root 

 assumes. Occasionally the tap-root divides 

 into two or three forks, as in the poisonous 

 Mandrake (Mandragora officinalis), where 

 they have a fancied resemblance to the 

 human form though this is not the 

 origin of the name of the plant. In days 

 of popular ignorance and credulity the 



Mandrake was looked upon with super- 

 FIG. 224,-Roo^criON OF YOUNG stitioug awe by all classes? and itg roots 



(Lettering explained in the text.) were said to be endowed with animal 



-re. 



