258 The Ohio Naturalist. [Vol. V, No. 3, 



called megasphorophylls and microsporophylls. There are num- 

 'Crous microsporophylls in each cone but usually only one meg- 

 asporophyll which is the lowest one of the set. The microsporo- 

 phyli 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 develop 

 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 Hetero- 

 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 



