STRUCTURE OF THE ROOT. 127 
sist entirely of parenchymatous cells (figs. 248, a, and 249, c); 
wood-cells and vessels (fig. 249, fv), however, soon make their 
appearance, and are constantly added to below by the new tissue 
formed as the root continues to lengthen. When the root is fully 
developed, these vessels and wood-cells generally form a central 
mass of wood (figs. 249, fv, and 250, f), in which there is com- 
monly no pith, and no medullary sheath, but the medullary rays 
exist as in the stem. Roots, however, differ from stems in the 
arrangement of the parts of their fibro-vascular bundles. Thus 
in roots, the phloém or liber portions alternate with the xylem or 
woody portions, instead of being placed external to them as in 
stems ; and some other minor differences also occur. Externally 
there is a true bark or cortex (fig. 250, 7, 7), which is also 
covered when young by a modified epidermis without stomata 
(fig. 128), and which, as we have seen, is sometimes called 
epiblema (page 60). This epidermis is also furnished with hair- 
like prolongations, which are termed root-hairs or fibrils (figs. 
128, and 248, c, c). The latter are especially evident upon young 
growing roots, and as these ad- 
vance in age they perish, while 
the tissue from which they were 
prolonged becomes at the same 
time harder and firmer, and is 
converted gradually into cork- 
tissue. 
Roots have no leaves, and 
normally no buds, hence they 
have no provision for regular 
ramification ; but they appear to 
divide and subdivide according 
to circumstances without any 
definite order; hence while the 
branches of the stem have a 
more or less symmetrical ar- 
rangement, as already described, 
those of the root are unsymme- 4 
trical. The branches of the root Fig. 250. Longitudinal section of the 
are also always developed endo- root of the common Bean (faba vul- 
Fic. 250. 
; garis), magnified five times. 7, 7. 
genously (fig. 250, n, 1), thatis, 
they are deep-seated, being de- 
rived from the pericambium (ig. 
247, pc) or outer layer of the 
Cortex of the main root. jf. Fibro- 
vascular bundles. , n, 2, . Lateral 
roots in different stages, developing 
from the pericambium and ultimately 
bursting through the cortex. h, h. 
Root-cap, or pileorhiza, of the lateral 
plerome or procambium. As they 
increase in length they ultimately 
push through the tissues which are superficial to them, namely, 
the cortical layers and epidermis of the main root, which are 
therefore not continuous with the similar tissues of the branches 
(fig. 250, 7, 7). The branches are thus merely repetitions of 
the original axis from which they are developed, and grow, 
roots. After Prantl. 
