Jan., 1905.] 
Heterosporous Pteridophyte. 
259 
sporocarp or modified leaflet containing the sporangia, after being 
soaked for some time in water, opens in a very peculiar way by 
the protrusion of a gelatinous ring containing the sori. This 
process represents an extreme specialization and adaptation to a 
semi-aquatic life. 
In Selaginella kraussiana the spores germinate sometime 
before they are discharged. The microspore has divided into two 
cells and the female gametophyte has developed to a considerable 
extent. This is a very interesting condition, since it represents 
one stage toward the development of seed plants in which the 
spores are not discharged and the gametophytes are completely 
parasitic in the sporangia. In some Selaginellas the cones with 
the sporangia and spores are shed. 
The gametophytes are very small especially the male. The 
vegetative part of the male thallus is represented practically by 
a single cell. The remaining cells developed inside of the spore 
wall represent the antheridium or spermary. In some of the 
Heterosporous Pteridophytes the antheridium breaks through the 
microspore wall, in others the spermatozoids escape through the 
break in the wall, no part of the male thallus protruding. The 
cells of the antheridium are differentiated into peripheral or wall 
cells and two masses of central cells from which the spermatozoids 
are developed. The spermatozoids are very small and have two 
flagella. In the quillworts and water-ferns the spermatozoids 
are multiciliate. Externally the male gametophytes of most 
Heterosporous Pteridophytes look very much like the pollen- 
grains of the seed plants with which they are homologous. 
The female gametophyte projects somewhat beyond the meg- 
aspore wall. It begins its development by free cell formation 
and later a layer of cells is formed in one side of the spore. This 
breaks through the spore wall and a number of archegonia or 
ovaries are produced, but the greater part of the spore cavity is 
filled with irregular vegetative cells. The female thallus is des- 
titute of chlorophyll and is dependent upon the food laid up in 
the spore. In Marsilea the female gametophyte is much more 
reduced having only a single archegonium whose neck projects 
from one end of the megaspore. In its older stages the female 
gametophyte may develop some chlorophyll. Each archegonium 
as usual contains a single oosphere or egg which has been formed 
by the division of a mother cell. The sister cell of the egg, called 
the ventral canal cell, alwa}''s dissolves. This small female 
gametophyte is homologous with the so-called embryosac of the 
seed plants. 
While the gametophytes are lying on damp ground, and when 
covered with water, fertilization is accomplished. In some 
Selaginellas, however, fertilization occurs while the megaspores 
containing the female gametophytes are still in the megasporan- 
