The Ohio Naturalist. 
[Vol. V, No. 3, 
258 
■called megasphorophylls and microsporophylls. There are num- 
erous microsporophylls in each cone but usually only one meg- 
asporophyll which is the lowest one of the set The niicrosporo- 
phyll has a single microsporangium on the upper side in the axil 
and the megasporophyll also produces but one megasporangium 
in the same position. In some of the other classes the two kinds 
of sporangia may be on the same leaf or leaf segment, which may 
be closed up like the ovulary of a carpel. The microsporangium 
and megasporangium produce the nonsexual spores, the micro- 
spores and the megaspores. The spores are produced in the fol- 
lowing manner: In the microsporangium there are a number of 
-cells called microsporocytes which become free in the cavity by 
the dissolution of their walls. Each microsporocyte divides 
twice giving rise to four cells. These cells develop into the 
microspores. During the first division of the microsporocyte the 
chromosomes of its nucleus are reduced so that the nuclei of the 
spores have only one-half as many chromosomes as were present 
in the nuclei of the sporophyte or nonsexual plant. The mega- 
sporangium also contains a number of megasporocytes which 
divide in the same way as the microsporocytes and form a spore 
tetrad. During the first of the two divisions the chromosomes 
are also reduced from the 2x number to x. The chromosomes 
are small bodies which appear in the nucleus during division and 
are probably special bearers of hereditary tendencies. These 
reduction divisions of the sporocytes are of great importance in 
the life cycle of the plant because of the profound changes which 
take place in the chromosomes. Only four megaspores develo]) 
and they become so large that the megasporangium bulges out 
and becomes somewhat four-lobed. In Marsilea only one mega- 
spore develops in each megasporangium. The difference in size 
between microspore and megaspore in Selaginella kraussiana is 
very great. The ratio in volume in the mature condition is often 
more than 1 : 2000. The small spore having only a limited 
amount of cytoplasm and very little room for food material, 
always produces a male gametophyte when it germinates; while 
the megaspore always produces a female gametophyte. The 
determination of the sex is apparently not primarily an inherited 
•character but depends on the environment during the early stages 
of embryonic development. Thus as stated above, the Iletero- 
sporous Pteridophytes by producing a difference of environment 
in the spore are able to keep the two sexes distinct. Since there 
are no hermaphrodite individuals there is no possibility of self- 
fertilization. This is true for all plants which have developed 
heterospory. The ripe sporangia open by vertical slits and the 
spores are thus discharged. In Azolla the microspores are imbed- 
ded in a foamy mass of substance called a massula on which 
peculiar anchor-like appendages are developed. In Marsilea the 
