Mie auc aei 
1900 | SPOROPHYLLS AND SPORANGIA OF ISOETES 249 
shows the vacuolated appearance of the sterile cells. Shortly 
afterwards the mother cells break away from the tapetum, which 
from this time on gains in density and apparent activity. The 
mother cells, at first in a continuous mass, soon break up into 
smaller and smaller groups of cells by the enlargement of the 
cavity in which they float. Finally the individual cells fall apart 
and round up, and pass rapidly through the two divisions by 
which the microspores are formed. 
No attempt was made to follow closely the cytology of these 
divisions because it was found impossible to make any satisfac- 
tory observations on the corresponding divisions of the mega- 
spore mother cells. The following notes may however be of 
interest. The achromatic figures appear to have a polycentric 
origin, and the chromatin passes through a synapsis stage. All 
the nuclei make their preparation for division and begin to 
divide almost simultaneously, and this notwithstanding their 
immense number. It is possible to find a better series of 
karyokinetic figures in a single sporangium of many ferns, 
where there are but sixteen mother cells, than in an Isoetes 
microsporangium where the mother cells number three or four 
times as many thousand. This 1 think may be regarded as an 
additional proof of the growth of the sporangium as a unit, and 
not as an aggregation of segments. 
In the majority of cases the two divisions are of the type 
Which is characteristic of cycads and monocotyledons, and has 
been called “successive :” that is, the first division of the nucleus 
is followed by the formation of a cell wall before the immedi- 
ately following division of the daughter nuclei (fig. 53). The 
Spores in this case are bilateral and may have their nuclei in one 
Plane or in two planes at right angles to each other. But it is 
not at all infrequent to find the divisions of the simultaneous 
type; that is, the first division of the nucleus is not attended by 
cell division, but before a wall is formed between the daughter 
cells each new nucleus begins its second division (jig. 54)- In 
this case the spores may be of the bilateral type, as in PE. 55 
@ and 4, or they may be tetrahedral as in fig. 55 ¢- Much 
