1900] LIFE HISTORY OF SILPHIUM 121 
and the cell organizing the root cap; it differs from Senecio in 
that this one cell contributes not only dermatogen and periblem, 
but also the terminal portion of the plerome. 
The possibility that the tier of cells called c, in embryos older 
than that in fig. 77, arose by the transverse division of the inner 
octants, has of course presented itself. But in such a case the 
cells above and below the dividing walls would show clear indi- 
cations of their being thus related, and the two tiers would be 
symmetrical as to number and position of cells on either side of 
the dividing line. Such a symmetry is illustrated in the lower 
octants of figs. 82 and 85, where two tiers of cells are in a process 
of separation. But no such symmetry is to be found between 
tier c and the tier above it, in any of the figures on plate V. 
These two tiers proceed at very different rates of growth, as a 
comparison of figs. 78 and 85 or 86 plainly demonstrates. More- 
over, in the study of such a stage as fig. 79, the longitudinal 
walls separating the octants are, we may say, in the plane of the 
paper and perpendicular to it, respectively ; whereas the corre- 
sponding walls in tier c always run in different planes, being 
frequently inclined as much as forty-five degrees to the longi- 
tudinal walls in the octants. These facts prove conclusively that 
tier ¢ has an origin independent of the octants, and that the 
description of its ultimate fate, as already given, is correct. 
There yet remains for us the consideration of the origin of 
the root cap. This point seems to have suffered from a general 
neglect. Hofmeister (1) gives a short description of Helianthus, 
but no detailed study of the sequence of cell divisions was made. 
The first account giving any reliable data upon the point in 
question is that of Hanstein (2), whose description of the process 
in Capsella has already been mentioned, and is too familiar to 
need repetition. Ina second mode of cap origin, described for 
nothera, a lenticular cell is cut out from the upper surface of 
the hypophysis cell, and becomes the periblem terminal; while 
the remaining portion produces the dermatogen of the root, by 
the splitting of which the cap arises. Hanstein does not describe 
any Composite embryos. 
