ROOT. 
163 
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 phloem-bundles lie in the intervals between the 
FIG. 122. — Lonsjitudinal section through the apex of a root of maize ; a a outer and older layers of the root-cap ; i i inner 
and younger layers; apex ; mgf\\\e. plerome ; becomes the pith, ^ a vessel, yxylem ; r the cortex which is produced 
from the periblem at the apex ; e e epidermis, continued into the dermatogen at the apex ; v 71 thickened outer wall of the 
epidermis (cuticle) ; (the origin of the root-cap from the dermatogen is not evident here ; the figure was drawn long before this 
discovery), 
primary vascular bundles at the circumference of the fibro-vascular cylinder (see 
Sects. 16, 18.) 
Although roots are commonly present in vascular plants, i. e. the higher 
Cryptogams and all Phanerogams, there occur even in these groups particular species 
from which they are entirely absent. Thus among Rhizocarpeae the genus Salvinia^ 
M 2 
