Igo1] SPORANGIA AND GAMETOPHYTES OF SELAGINELLA 181 
3. The tapetum is formed in part from the sporogenous cells 
near the exterior of the mass, and in part from adjacent vegeta- 
tive cells which come to form a more or less regular layer. It 
is defined very early, and is active and glandular up to the 
period that the spores have attained their full size, then becomes 
reduced to a thin epithelial-like layer lying against the spo- 
rangium wall. 
4. Microsporangia and megasporangia are indistinguishable 
before the spore mother cells are differentiated. At this stage, 
in the case of the megasporangium, one or rarely two cells 
become more conspicuous and divide into spores. If micro- 
Spores are to be formed, the majority of the cells continue 
dividing. After the megaspore mother cell is differentiated, all 
other cells in the megasporangium cease dividing. 
5. In S. apus four megaspores arranged tetrahedrally are 
formed within the spore mother cell. In S. rupestris four spores 
may develop in the same manner, only one or two of which may 
come to maturity; or, most commonly, there may be but a single 
division of the mother cell nucleus, in which case there are but 
two spores formed; or again, there may be a redivision of one 
only of the two daughter nuclei, resulting in three spores of 
which only one or two attain maturity. Isolated cases have 
been met with where but one megaspore was formed. As there 
were no signs of other aborted spores, presumably the mega- 
Spore mother cell never divided, but became directly the single 
megaspore. In no case have more than two spores in a single 
megasporangium been found which were fertile, and very fre- 
quently only one. 
6. The megaspore of each species has three distinct coats, 
the €xospore, mesospore, and endospore. The former originates 
on the inner face of the spore mother cell membrane, and when 
first distinguishable is a film of unequal thickness. This either 
directly or indirectly gives rise to the exospore. A thick layer 
develops between the exospore and the protoplasmic vesicle, 
which later in its history separates into two layers, the meso- 
Spore and the endospore. 
