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 

 bv the division of a mother cell. The sister cell of the egg, called 

 the ventral canal cell, always 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, fertiUzation'occurs while the megaspores 

 containing the female gametophytes are still in the megasporan- 



