i88 THE STRUCTURE OF ECONOMIC PLANTS 



tiated consisting of one to five, usually two, large scalariform ves- 

 sels; and when maturation of the stele is complete, there are no 

 fundamental parenchyma cells remaining except the single-layered 

 pericycle. Casparian strips are laid down early on the radial and 

 end walls of the endodermal cells and are well developed in seed- 

 lings a week old. The cortical parenchyma consists of six or seven 

 layers of thin-walled cells with intercellular spaces at their angles, 

 and the epidermis is a single layer of cells, many of which elongate 

 radially to form hairs. 



The development of the root is similar to "type x" as described 

 by Janczewski (8) and modified by Treub (2.1). In this type, the 

 meristem consists of two histogens, a well-defined plerome which 

 gives rise to the stele, and, overlying it, a group of common initial 

 cells two layers in thickness from which originate the cells of the 

 root cap, epidermis and cortex. The plerome consists of a layer of 

 a few cells from which the pericycle is first differentiated. Then 

 as the protoxylem is differentiating, the cells of a centrally located 

 axial row enlarge, especially in the longitudinal direction, without 

 further division; while the adjacent cells continue to divide. 

 These large cells finally mature as segments of a metaxylem vessel 

 or vessels. (Fig. 87, C, D.) 



One or two periclinal divisions of the undifferentiated cells 

 enlarge the stele so that it is from five to ten, most commonly 

 seven, cells in diameter. The pericyclic cells may be somewhat 

 shorter than the adjacent endodermal and cortical cells, and can be 

 distinguished from the provascular cells which lie immediately 

 centrad to them by the small caliber of the latter. 



Distal to the plerome is a meristematic region three to six cells 

 in diameter and two to four cells in depth. The cells of the inner 

 layer or layers of this region function as a periblem and produce the 

 cortex, which reaches its mature width as a result of one or more 

 periclinal divisions. The cells in this region divide in the trans- 

 verse plane more frequently than do those of the stele; and less so 

 than the epidermal cells, so that they are intermediate in length. 

 The lateral cells of this same meristematic region produce a single 

 layer of epidermal cells which divide only anticlinally. The outer- 

 most layer of the meristematic region cuts off the cells of the root 

 cap. This consists of several dome-shaped layers of cells overlying 

 one another, the outermost being the oldest and largest. At the 

 apex of each layer, the cells are larger, slightly more numerous, and 



