50 8 Tansley and Chick . — On the Structure 
ture, as representing the current state of evolution of the plant 
body, naturally forming the starting-point of the investigation. 
Differentiation of Tissues behind the Apex. 
The further development and differentiation of tissues below 
the point at which the initials x. and ph. are derived from mi. 
can best be followed in a series of transverse sections (Figs. 
11-13). From these it is seen that the radial and tangential 
divisions in the layer of xylem- and phloem-initials is by no 
means constant and regular. The new longitudinal walls are 
formed in various orders, and division is much further advanced 
at some places on the circumference of the stele than it is at 
others. The three zones of stem-tissue derived respectively 
from o., m., and A, are extremely obvious in Fig. 11. The 
cortical and pith-cells have thicker and darker walls, the nuclei 
of the former and some of the latter (those to the left) having 
already lost their meristematic appearance, while the inter- 
mediate zone comprising the initials of the vascular ring and 
sheath-layers is still markedly meristematic. A stage in which 
there are twice as many xylem- and twice as many phloem- 
initials as there are pericyclic cells is common, and there are 
often no further radial divisions in the initials of the vascular 
ring, but tangential divisions continue irregularly till the ring 
is four or five cells thick. In spite of this frequent irregularity 
in division, the common origin of a given endodermal and 
pericyclic cell with the adjacent xylem- and phloem-initials is 
often very obvious (e. g. at A in Fig. 12). The endodermis 
differentiates before the other tissues derived from m., early 
acquiring the mucilaginous contents which give it such charac- 
teristic staining reactions. Meanwhile the first sieve-tubes are 
developed from the outermost phloem-initials at scattered 
spots on the circumference of the stele (Fig. 12). This dif- 
ferentiation continues till there is a fairly complete ring of 
phloem, and then isolated tracheids begin to develop abutting 
on the pith (Fig. 13). Hence the xylem is technically endarch , 
though whether this has any morphological significance is 
perhaps doubtful. It may, however, be noted that the stem- 
