i8o 



PART II. VEGETABLE HISTOLOGY. 



The Epidermis. — This may be present, or it may be early 

 replaced by the growth of the sublying tissue of the cortex, 

 particularly by the formation of cork cells. 



When present, the tissue is substantially the same as that 

 already described under epidermal tissue. Stomata, however, 

 are never present. 



Root -#"#*>.$•.— Beginning a short distance above the root-cap, 

 and extending a little way up the rootlet, the epidermis, or the 

 superficial layer of the cortex 

 which replaces it, is densely 

 covered with root-hairs. The 

 character and function of these 

 hairs have already been de- 

 scribed. As the root lengthens, 

 the old hairs above wither and 

 disappear, while new ones are 

 continually formed below. 



The Cortex. — This varies 

 greatly in different plants. It 

 may be thick or thin ; it may 

 be made up almost wholly of 

 parenchyma, or it may contain 

 a variet} r of tissues, such as col- 

 lenchyma, sclerenchyma, cork, 

 fibrous tissue, secretion cells, 

 and laticiferous ducts. 



It is usually, however, less complex in its structure than the 

 cortex of the stem. The inner layer of the cortex constitutes 

 the endodermis, which ensheaths the central cylinder. 



The Central Cylinder. — This, in the root of Club-mosses, Ferns 

 and nearly all flowering-plants, consists at first of a radial fibro- 

 vascular bundle, Figs. 438 and 439. 



The number of radiating masses of xylem and phloem, in the 

 central cylinder, varies greatly in different plants. In the Cru- 

 cifera there are usually two of each ; in the root of the Castor-oil 

 plant, four ; in that of the Bean, five ; in that of the Beech, eight; 

 in that of Amaryllis, about twenty-one; and. in that of Indian 

 Corn as many as fifty. In general, the rays in the roots of dicotyle- 

 dons are few, while in monocotyledons they are usually numerous. 



Fig. 440. — Diagram of root-tip as it ap- 

 pears in longitudinal section, illustrating 

 the different regions, a. the growing point; 

 6 6, the root-cap; cc, imperfectly developed 

 epidermis; d d, imperfectly developed pri- 

 mary cortex ; e, imperfectly formed central 

 cylinder. Much magnified. 



