SAAR 
1897] THE LIFE HISTORY OF SALIX 163 
to be differentiated, the first step in this differentiation being 
marked by the spindle in jig. 77. This division completes 
the dermatogen of the root tip, joining it with the dermatogen 
of the rest of the embryo, and furnishing the first layer of the 
root cap (figs. 72, 75, 77a). These figures show no differentia- 
tion into periblem and plerome. I do not believe that the sus- 
pensor contributes anything to the periblem in Salix. An 
embryo almost in the cotyledon stage (fig. 74) shows a complete 
dermatogen, but still no definite plerome and periblem. Nearly 
mature embryos ( fig. 66) have the periblem and plerome sharply 
differentiated a short distance above the dermatogen of the root 
cap, but are indistinguishable at the apex, and both tissues still 
come froma common meristem. This figure represents the char- 
acters of the various regions of late embryos. The plerome cells 
are marked by dense protoplasmic contents free from vacuoles. 
Except very near the meristem they are elongated, and their 
long nuclei usually have two or more nucleoli. It is a region of 
cell elongation rather than of cell division. The periblem cells 
with their numerous vacuoles, spherical nuclei, and looser arrange- 
ment, present a noticeable contrast, which is emphasized by the 
fact that they are broader than long, and show evidences of cell 
multiplication rather than elongation. The prevailing divisions 
are transverse. The cells of the hypodermal layer of the peri- 
blem soon become sharply differentiated. The protoplasm with 
its nucleus is crowded against the inner wall of the cell by the 
encroaching vacuoles, which merge into one large vacuole con- 
taining a substance which seems to be suberin. A transverse 
section of the plerome and part of the periblem at this stage is 
represented in fig. 69. In fig. 76 the periblem and plerome seem 
to be completely differentiated. At the apex there is only one 
layer of periblem between the plerome and dermatogen and this 
is usually the case in mature embryos. This figure also shows 
the usual appearance of the layers of the root cap. The root 
region of an embryo which has completed its intraseminal 
development has a separate meristem for the periblem and pler- 
— ome Y Sig. oe hee | and ewes being shaded, and 
