16 THE FLOWERING PLANT. 
spherical, but many-sided (2.e, polyhedral), a natural result of 
growing together in numbers. This point may be illustrated by 
pressing together soft clay pellets, when the spaces between them 
get filled up, and a many-sided form is consequently acquired. 
Mutual pressure is similarly exerted by growing cells owing to 
their turgidity (p. 9), and a spherical shape cannot therefore 
be assumed. In sections of parenchyma the constituent cells of 
course appear polygonal in outline. The cell-walls are of cellu- 
lose, and protoplasm with nucleus is present, with cell-sap in 
addition. 
The vascular cylinder is chiefly made up of the vascular system 
of tissue, which is the most complex. It is composed not only of 
cells, but also of cell-derivates, z.e., structures derived from cells. 
In this case, the most important of them are tubular in nature, 
and may be broadly spoken of as vessels ; hence the term “‘vascular” 
tissue. Their structures will be dealt with later on (pp. 31 and 34). 
It is only necessary here to emphasize the fact that even in very 
young roots they are aggregated into a central “ cylinder.” 
All roots are capable of increasing in length. Kpidermis, cortex, 
and vascular cylinder gradually become more indistinct as the 
apex of the root is reached, and ultimately merge into a minute 
mass of small cells, which constitute the growing point. ‘Their 
walls are extremely thin and their protoplasm abundant. New 
cells are continually being formed by divisions somewhat as 
described on p. 11. Here, however, there is no rounding off, 
but those cells which are nearest the older parts of the root 
increase in size and alter in shape, ultimately becoming mature 
cells of the epidermis, &c., &c. The growing-point is not at the 
actual apex of the root. If it were so, its delicate cells would be 
exposed to constant injury from the hard particles of earth, 
against which they would be forced by the growth in length. 
The extreme end is occupied by a thimble-like sheath, the root- 
cap, which is made up of numerous layers of flattened cells, and 
covers over the growing-point, protecting it from injury (figs. 2 
and 4). As the outer layers of the root-cap are worn away, new 
inner layers are added by the growing-point. All roots are char- 
acterized by the presence of a root-cap, and it is present even in 
water-roots, as may be ‘well seen by examining those of the duck- 
weed under a low power of the compound microscope. As a root 
increases in length, branches may arise from it. These are deve- 
loped in regular order, the youngest being nearest the growing- 
point (fig. 2). Technically described, they arise in acropetal suc- 
cession. Adventitious roots are an exception to this. 
Some roots increase not only in length but in thickness as well. 
This increase is effected by a cambium layer, situated within the 
