DICOTYLEDONOUS ROOT. 169 



a cross-section through a lateral root which is about 1 mm. thick, 

 and at perhaps f- inch from the tip. 1 The surface is composed 

 of a parenchymatous cortex at least ten cells thick. The outer- 

 most cell-layer of the cortex is not sharply denned, as a differen- 

 tiated epidermis is wanting. The centre of the section is occupied 

 by the central cylinder. This is surrounded by the endodermis, 

 which consists of flat, thin-walled, corky cells, whose walls are 

 brown, and whose diameter is manifestly inferior to that of the 

 cortical cells. These cells show, on the radial walls, the charac- 

 teristic dark shadings. Around the endodermis is developed a 

 " strengthening 1 ay er^ r like wise unilamellar ; its cells have the 

 width of the other cortical cells, but have, however, their radial 

 walls distinguished by a thick, shining, yellow ring. These 

 annular thickenings coincide in adjoining cells, and therefore 

 give in cross-section the figure of a very bi-convex lens. The 

 central cylinder shows a diarch xylem in the form of a transverse 

 plate (Fig. 68 t'); composed at its two ends of the narrow spiral 

 vessels, and, for the rest, a band of broader tracheides, with 

 bordered pits, of the kind characteristic of Coniferae. On each 

 side of the tracheides lies a layer, in the main double, of ground- 

 tissue (conjunctive) cells, with narrow cavities, thin walls, and 

 containing starch. To these adjoin the still smaller-celled tissue 

 of the thin-walled bast, composed of sieve-tubes and protoplasmic 

 bast-parenchyma. Lastly outside this bast are the starch-contain- 

 ing cells. 



Now examine a cross-section about 1'3 mm. thick, and at about 

 1 inch from the root-tip, and we shall see that, on both sides of 

 the plate of tracheides, the layer of the conjunctive ground-tissue, 

 bounding the bast internally, has commenced to divide. It is 

 converted into a layer of cambium, which henceforth produces 

 internally tracheides, externally bast, and on both sides medullary 

 rays. We shall see the results of the continued activity of this 

 cambium layer in a root 2 mm. thick ; the present structure 

 is illustrated in the subjoined Figure 68. The cross-section 

 shows the general relations already known to us ; the cortex (c), 

 the outer strengthening layer (m), the endodermis (e), and the 

 central cylinder. The outermost cell-layer of the pericycle has 

 in the meantime begun to divide by tangential walls, and forms a 

 thin layer of periderm adjoining the endodermis. On both sides 



1 Seedling plants serve very well. [Eo.] 



