ROOT. j5^ 



Roots occur only in those plants the tissue of which is traversed by fibro- 

 vascular bundles, and they themselves therefore always contain fibro-vascular 

 bundles ; but these latter differ from those of the stem and leaves in the first 

 vessels being formed near the circumference of the bundle, while the later 

 ones are always formed further inside, and hence centripetally in reference to 

 the diameter of the root. The phlofe'm-bundles lie in the intervals between the 



FIG. 122.— Longitudinal section thfough the apex of a root of maize ; a a outer and oldef layers of the root-cap ! i' z Itiner 

 and younger layers; j apex ; wz^ytlie plerome ; w becomes the pith, ^ a vessel, ^xylem ; ;f 7- the cortex which is produced 

 from the periblem at the apex; ee epidermis, continued into the dermatogen at the apex; vv thickened outer wall of the 

 epidermis (cuticle) ; (tlie origin of the root-cap from the dermatogen is not evident here; the figure was drawn lohg before this 

 discovery). 



primary vascular bundles at the circumference of the fibro-vasculaf cylinder (see 

 Sects. 16. 18.) 



Although roots are commonly present in vascular plants, t. e, the higher 

 Cryptogams and all Phanerogams, there occur even in these groups particular species 

 from which they are entirely absent. Thus among Rhizocarpese the genus Salviniaf 



M 2 



