THE ANATOMY OF THE ROOT 



47 



periblem at the root apex consists of an initial cell or a single 

 layer of a few cells which later becomes several layered by peri- 

 clinal divisions. A variation occurs in Linum in which the peri- 

 blem consists of two layers of cells overlying the plerome. The 

 outermost layer divides only anticlinally to form the outer layer 

 of the cortex or hypodermis, and the inner layer divides in all planes 

 to form the remainder of the cortex. Type (3) is probably the most 

 common one in dicotyledons; and occurs, among others, in Rapha- 

 nus, Ipomoea, Lactuca, 

 and Linum. (Fig. 16.) 



In type (4) there is a 

 common meristematic 

 zone which extends 

 across the apex of the 

 root, and sharply de- 

 limited histogens are 

 lacking. The peripheral 

 cells of this .terminal 

 meristem enlarge and 

 mature as the root cap. 

 The stele and cortex 

 arise from derivatives of 

 the centrally located 

 cells, and the epidermis 

 is differentiated from the 

 laterally placed ones. 

 This type is frequently 

 referred to as the open 

 type meristem. The Cu- 

 curbitaceae (Cucurbita) 



and Leguminosae (Pisum, Phaseolus, and Medicago) are represen- 

 tative of this type. (Fig. 17.) 



Irrespective of the type of meristematic organization, growth 

 and differentiation occur in varying degrees; and there is an early 

 blocking out of the epidermal, cortical, and stelar regions. In 

 ontogeny, the cells of each primary tissue soon reach maturity, and 

 the stelar pattern is laid down conforming to one of the protoste- 

 lic or siphonostelic types noted above. 



The dynamic aspect of root development is clear when successive 

 transections of a root are studied through the regions of elongation 



Fig. 17. Longisection of the apex of the root of Pisum 

 showing the character of the terminal meristem : a, epi- 

 dermis; b, pericycle limiting the stele; c, correx; d, 

 root cap; e, meristem. (After Janczewski, Ann. Set. 



Nat.') 



