18 STRUCTURE OF ROOTS. 



and these organs are almost always buried in the earth. Some- 

 limes the roots float freely in the water, and there are some 

 plants that insinuate them into cracks in walls, or in crevices 

 of the stem of some other plant, as the mosses, for example. 

 There are certain plants, the roots of which arise at a considera- 

 ble distance above the surface of the soil, and have only their ex- 

 tremity buried in the earth, so that the greater part of their length 

 remains exposed to the air. To such roots we give the name of 

 aerial or adventitiovs roots; the maize or Indian corn and many 

 other American plants have them. 



6. We see now that it is not a constant character of roots to 

 be covered up in the earth ; and, on the other hand, we should be 

 equally deceived if we were to regard as roots all parts of plants 

 that are buried in the soil ; for it sometimes happens that the 

 stem, instead of rising up through the air, creeps horizontally 

 under ground ; but the structure of the two parts is different, and 

 prevents them from being confounded with each other. The tissue 

 of roots is whitish, and never becomes green by exposure to the 

 action of light, which occur to all other parts of plants. [Those 

 stems which creep along under the ground, are called root-stalks, 

 or subterranean, or rhizome (from the Greek ridsa, root) stems ; 

 the stems of the orris root, ginger, and potato, upon which grow 

 the tubers we eat, are instances of this kind.] 



7. The root, considered as a whole, generally consists of 

 three distinct parts: First, the body or middle part, which is 

 sometimes globular, and, at others, similar in form to a descend- 

 ing stem ; Second, the radicles, the more or less delicate fibres 

 which terminate the root at its lower part ; and, third, the neck 

 or collum, the point that separates it from the stem, and which is 

 often marked by being smaller. 



8. The internal structure of roots varies ; in general, it is di- 

 vided into the cortical part, or bark of the root, and central or 

 ligneous part. 



9. The bark of the root, which is often very thick, is entirely 

 composed of cells ; its epidermis is always without sto'mata. 



10. The ligneous body of the root is not ordinarily composed 

 of distinct fibres, and we do not find tracheae in it as in the stalk 

 or stem of vascular plants ; nor has it pith in the centre. 



6. Are roots always under ground ? Does the stem ever grow under 

 ground ? How is a root distinguished from a stem that grows under 

 ground ? How is the tissue of roots characterized ? How are those sterna 

 which grow under the soil designated ? 



7. How is the root divided ? 



8. How is the internal structure of roots divided ? 



9. What is the structure of the bark of the root? 

 lu. What is the ligneous body of the root '( 



