ROOT APEX OF THUJA. 223 



Thuja, and of Gymnosperms generally, consists of the outer layers 

 of the periblem ; dermatogen and calyptrogen are wanting. The 

 initial layers of the periblem, passing over the apex of the 

 plerome, divide by periclinal and anticlinal walls. The periclinal 

 divisions increase the number of layers of the periblem, and 

 replace from the interior the elements which are exfoliated from 

 the periphery. The anticlinal walls increase the number of cells 

 in the individual layers, and provide chiefly for the formation 

 of the cortex. As the anticlinal walls in successive layers corres- 

 pond pretty regularly with one another, they form anticlinal rows 

 of cells, which, straight in the interior, separate from one another 

 externally, like the component rays of spray which collectively 

 constitute the jet issuing from a fountain, forming therefore a 

 constantly-extending series of co-axial parabolae. The periclinal 

 divisions in the initial layers of the apex have as result that the 

 cell-rows of the cortex, when these are followed towards the 

 point, appear constantly doubled. The most median, straight, 

 anticlinal rows of cells in the periblem of the root-apex are dis- 

 tinguishable from their neighbours. They form a " periblemic 

 column " (Periblemsaule), which is lost in the outermost, brown 

 elements of the root-cap. This column appears clearer, its cells 

 immediately adjoin one another, while those bordering laterally 

 form air-containing intercellular spaces. Moreover, the cells of 

 the column are distinguished by especial richness in starch. 

 As results from the foregoing relations, the root of Thuja can 

 possess no epidermis, the outer surface of the root being com- 

 posed of the, for the time being, outermost layer of the periblem. 

 If such a layer is followed in the direction of the apex, we shall 

 soon see it pass under another, which now for a time constitutes 

 the surface. The outermost living layers of cells are protected at 

 their surface by the collapsed walls (become brown) of the dis- 

 organised layers of cells. The roots of the Gymnosperms have, 

 in general, no root-hairs ; we search for such in vain in Thuja 

 occidentalis. 



The adjoining figure, 88, gives, with low magnification, the 

 structure of a longitudinal section, and will help us to understand 

 it. Naturally, with such a low magnification, the arrangement of 

 the cells can only be indicated. Passing from the exterior 

 towards the interior, we see, therefore, the brown, collapsed 

 covering of cells (x) ; then the periblem (pr), wMch can be traced 

 over the apex of the root, and whose outermost layers form the 



