DEVELOPMENT OF THE EOOT. 597 



growing-point is also destitute of epiderrn, but its place is supplied by a 

 layer produced from the same tissue as that from which the root-cap is 

 formed: this form is common among Dicotyledons. 4. The primary 

 tissues are not distinctly defined, except along a transverse layer of growing 

 cells or generating layer : this type is also met with in Dicotyledons 

 (Legumiriosse, Cucurbitaceae), &c. 5. The root consists of two tissues 

 only, the central cylinder and the cortex, which latter fulfils the office of 

 the* root-cap : to this last type belong the roots of Gymnosperms j the 

 root-cap as a primary tissue has no existence in these plants. 



Root-cap. The root-cap is formed from a layer of tissue distinct from 

 that constituting the body of the root (p. 536, tig. 583). It grows (except 

 in the first type) by means of a generating layer, which is cast off when 

 its functions are completed, with the rest of the dead portions of the root- 

 cap, as in the second type, or is immediately transformed into epidermis, 

 as in the third and fourth types. In Gymnosperms, as has been already 

 stated, the true root-cap, as an independent tissue, does not exist (fifth 

 type). In Vascular Cryptogams the new layers of the root-cap are derived 

 from the transverse division of the solitary terminal cell. 



Epidermis. This is generally indistinct, and may be altogether want- 

 ing, as in Gymnosperms. It is rarely independent (first type). It arises 

 from the transformation of the outer cortical layer (second and fifth types), 

 or from the transformation of the generating layer, as in the third and 

 fourth types. 



In the Vascular Cryptogams, the epidermis becomes differentiated after 

 the central cylinder, and does not divide into different layers, except in 

 the case of some Ferns. 



The Primary Cortex is a tissue always distinctly separated from the 

 central cylinder. It consists at the apex of the root of a single layer made 

 up of a very small number of cells. Further from the apex the cells di- 

 vide and subdivide, so that several layers are there formed, generally in a 

 centripetal direction. 



The Outer Cortex of collenchyrnatous cells is always a secondary forma- 

 tion, either from the outer portion of the primary cortex (Hydrocnarit\ or 

 it results from the development of the cells beneath the epidermis in a 

 centrifugal manner (Stratiotes). 



In the fifth type the cortex is thicker at the apex than at the periphery 

 of the root ; in the fourth type it is derived from a transverse generating 

 zone ; so that the mode of growth of these two types is quite different from 

 what it is in the three preceding ones. 



In Vascular Cryptogams the zone from which the cortex originates 

 divides into two layers ; the outer forms the outer cortex by repeated sub- 

 division in a centrifugal direction, while the inner layer consists of cells 

 developed centripetally. Intercellular spaces only occur between the cells 

 of the inner layer of the cortex, and not between those of the outer layer. 

 The root of Vascular Cryptogams resembles that of Stratiotes in its two 

 cortical layers. 



The Central Cylinder is composed in the three first types of an axile 

 vascular bundle and a peripheral zone. In the two last types the cylinder 

 is homogeneous at the summit. The pericambium is always early 

 distinctly defined, and consists generally of a single, rarely of two or more 

 layers of cells. While, then, the growth of the root of Vascular Crypto- 



