PTERIDOPHYTA 



311 



arranged in a spike. In most species, the oldest (lowest) sporan- 

 gium contains four very large macrospores ; the others, many small 

 microspores. 



The Gametophyte 



Male Gametophyte. At the time the microspores are shed, there has already 

 been cut off from the body of the spore a small sterile cell (Fig. 276, x). The 

 large cell now undergoes repeated divisions, resulting in a single antheridium, 



B 



FIG. 277. Selaginella Kraussiana. Female gametophyte and embryo. A , B, sections 

 of germinating macrospore. A, with free nuclei, n. B, showing first cell-forma- 

 tion (X 350) ; per, spore-membrane. C, section of fully developed gametophyte, 

 with young embryo (X 200). D-G, development of archegonium (x 350). H, I, 

 young embryos (x 350) ; s, suspensor. 



consisting of a central mass of sperm-cells, and a layer of peripheral cells, which 

 are finally broken down. The minute spermatozoids are biciliate like those of 

 Lycopodium. 



Female Gametophyte. The macrospore begins its germination while still 

 within the sporangium, in this respect approaching the condition found in the 

 Seed-plants. The young macrospore contains a single, relatively small nucleus 

 and very little cytoplasm, most of its cavity being filled with transparent cell-sap. 

 As the spore enlarges, the protoplast becomes entirely free from the wall, and the 

 nucleus, surrounded by the contracted cytoplasmic membrane, has the appear- 

 ance of a large nucleus with a nucleolus. As the nucleus divides, the cytoplasm 



