1900 | SPOROPHYLLS AND SPORANGIA OF ISOETES 237 
cells which form the earliest rudiment of the sporangium fre- 
quently take a distinctive cytoplasmic staining, especially in 
material fixed in Flemming’s solution. It must be confessed 
that this means of recognizing sporogenous tissue is not so trust- 
worthy as one could wish, for at this period of the leaf’s history 
all the tissues are meristematic, and hence readily susceptible to 
protoplasmic stains. One who studies the origin of sporangia 
in Lycopodium or Selaginella meets with the same difficulty in 
those plants, a difficulty in my experience quite as great in these 
cases as in Isoetes. When the superficial layer of the sporan- 
gium has assumed its character as an epidermis, the deeper lying 
sporogenous cells are easily distinguishable by stain reactions 
from the surrounding tissues. At a later period the spore 
mother cells selected out of the general internal mass of the 
sporangium become quite distinct on account of their denser con- 
tents and more intense-staining. 
Longitudinal sections of young leaves show no space between 
the base of the ligule and the stem. At this time there is still 
an active uplifting of cells above the general stem level, a con- 
tinuance of the process by which the leaf first emerged. When 
the ligule has grown sufficiently to contain eight or ten cells in 
longitudinal section the space below it is occupied by one large 
cell with dense cytoplasmic contents (fig. 26). The next change 
which takes place is a transverse division of this cell as shown 
in figs. 27, 28. Comparisons of successive serial sections 
show that the two cells shaded in fig. 27 form the middle of a 
Sroup of cells arranged transversely to the leaf. This group of 
cells, distinguishable in good preparations by their deeper stain- 
ing and larger nuclei, constitute the rudiment of the sporangium. 
In order to learn its extent and arrangement recourse must be 
had to transverse and tangential sections. 
Most transverse sections of this early stage of the sporangium 
show that it is five cells in width. .Whether or not these can be 
traced back to a still smaller number I am in doubt. fig. 29 
certainly shows an example where the transverse Tow consists of 
only three cells, and it is clear that the shaded cells of fig. 3° 
