Te. 
1900 | SPOROPHYLLS AND SPORANGIA OF ISOETES 241 
their outer walls remain straight and become thicker than those 
which subsequently appear within them; and the incomplete 
separation of the derivatives of any single primary sperm cell 
from one another and their complete separation from those of 
other primary cells are shown by their dividing concurrently. 
I have frequently observed in the antheridia of Polytrichum, 
Porella, Marchantia, and Asterella that all the cells derived 
from one of the primary sperm cells enter into karyokinesis 
together, finish their division, and enter into the resting condi- 
tion together, quite independently of what may be going on in 
the derivatives of other primary cells. In such cases it is quite 
proper to speak of an independent growth; for the separation 
and isolation of each group by thickened walls are sufficient to 
insure a simultaneous exposure and obedience of all the cells to 
the physiological stimulus which induces karyokinesis. 
Are there any indications of such independent growth in the 
Sporangium of Isoetes? I can find none, either in the arrange- 
ment of the tissues or in the presence of thickened walls which 
mark the boundaries of the original archesporial cells, or in the 
simultaneous entrance of the cells of each group into the phases 
of division. All the mature cell walls of a growing sporangium 
are of equal thickness; and in marked contrast to what is seen 
in the leaves there is no regularity of stratification or lining-up 
of the cells. I am forced to conclude that the sporangium 
of Isoetes (at least of 7. echinospora and I. Engelmanni), just as 
the microsporangium of angiosperms, grows as a unit and not as 
a number of individual segments. 
Before continuing the subject of the development of the 
Sporangium it will be convenient to consider the formation of 
the velum. The velum makes its appearance very early in the 
history of the sporangium, almost as soon in fact as the first 
Periclinal divisions of the superficial cells. It is formed immedi- 
ately below the ligule. Hofmeister (1) says: ‘‘Of the two cells 
into which by a transverse septum the cell underneath the place 
of insertion of the ligule is divided the upper one becomes the 
Primary cell of the velum and the lower the primary mother cell 
