8o Harshberger. — Maize 



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cap of loose dead cells is constantly renewed from active lay- 

 ers within, so that the cap can be divided into two portions, 

 the outer dead, the inner active layers. The dermatogen, 

 continuous with the epidermis or piliferous layer of the root 

 above, is covered near the apex by the actively dividing 

 calyptrogen. The periblem occupies an intermediate posi- 

 tion between the dermatogen or proto-epidermis and the 

 plerome cylinder inside. The three meristematic layers, der- 

 matogen, periblem and plerome, take part in the construction 

 of the root, while the root cap is added as a fourth protective 

 element at the growing point. The central bundles are 

 formed from the plerome ; the calyptra is formed from the 

 calyptrogen. 



A cross section of the primary root shows the tissues in a 

 different aspect. Two regions are defined as the central 

 axis cylinder and the encircling parenchymatous tissue. The 

 cylinder's centre is occupied by woody elements with radiat- 

 ing arms reaching the pericycle ; the phloem portions alter- 

 nating with the xylem wings. The bundle system is a radial 

 one. Large dotted or scalariform ducts are prominent 

 elements, readily identified. Spiral tracheids are found 

 between the large ducts and the smaller external annular 

 tracheids. The sieve portion of the phloem is obscure, except 

 when the wood elements are reduced to a minimum. The 

 cell walls of the pericycle show scleroid thickenings. The 

 endodermis is distinguished by its position on the circumfer- 

 ence of the axis cylinder. The remaining parenchyma is 

 irregular with large intercellular air cavities. The primary 

 root soon disappears, as in monocotyledons generally, and is 

 replaced functionally by the secondary roots, which differ in 

 the arrangement of some of the more important elements. 



The layers in the secondary root are arranged as follows 

 (Fig. i, Plate XVI) : The large vessels of the bundle are 

 uniform in size ; the annular ducts and spiral tracheids have 

 increased until they occupy a large part of the space between 

 the pericycle and the wide dotted vessels ; the phloem areas 

 are small (Fig. I, P) ; the pericycle is formed of irregular 

 elliptical cells with long diameter anticlinal ; the endodermis 



