506 Tansley and Chick . — On the Str torture 
a periclinal wall into an outer segment ( o .) and an inner (i.). 
The latter then divides by another periclinal wall giving rise 
to an innermost cell (i'.) and a middle cell (mi) of the anticlinal 
series of three into which the original segment has now 
divided. Of these o. is a cortical initial, m. an initial of the 
endodermis, pericycle, phloem and xylem, and i r . an initial of 
the pith. The divisions of these initials are not entirely 
constant. The outer cell may divide at once anticlinally, or 
it may remain undivided for a time and then divide periclinally 
(commonly into two and almost immediately into four). The 
middle cell divides by a periclinal wall, which in most cases 
separates an initial (me.) of the endodermis and pericycle from 
an initial (mi) of the xylem and phloem. The innermost cell 
undergoes various divisions, largely horizontal, and the inner- 
most daughter-cells differentiate very shortly into pith-cells, 
which are formed so early that they are often only separated 
from the apical by two or three meristematic cells 1 . 
To return to the fate of the segments of the middle cell (m .) : 
on a level with the young pith, at a distance of -14 mm., and 
separated by about six cells from the apical in the apex figured 
in Fig. 10, the inner (mi) of its two segments divides by a 
periclinal wall into two narrow cells, elongated in the direction 
of the axis of the stem, the inner of which (x.) is a xylem 
initial, and the outer (ph) a phloem-initial. Both of these 
elongate, divide by radial and tangential walls, and very soon 
give rise to tracheids and sieve-tubes. Shortly after the 
division of mi. into ;tr. and//£., the outer segment (me.) divides 
tangentially into e. and /., initials of the endodermis and 
pericycle respectively. Outside these the cortex is now about 
eight cells thick, but is often disturbed by a leaf-rudiment. 
Detailed information as to the course of histogenesis at the 
1 The appearance of the resting nuclei of the pith-cells is quite different from 
that of the meristematic cells which give rise to them. The nuclei are smaller 
and look more homogeneous in our material (fixed in ordinary methylated spirit). 
They take up haematoxylin much less readily than do those of the meristematic 
cells, in which chromatin granules are quite obvious. Later on, however, the 
pith-cells divide again to a certain extent and their nuclei reacquire the meriste- 
matic appearance. 
