PTERIDOPHYTA 



291 



macrospore. The spermatozoids are coiled, the coils being numerous in Mar- 

 silia, where all but the lower larger coils have been shown to be derived from the 

 blepharoplast. The uppermost poils have no cilia. 



Female Gametophyte. The macrospore (Fig. 254) has the nucleus lying at 

 the upper end, surrounded by cytoplasm, which is free from the large starch- 

 grains found in the body of the spore. In Marsilia, the nucleus lies in a pro- 

 tuberance at the apex of the spore. 



The first division in the spore usually, but not always, separates this papilla 

 from the body of the spore, whose nucleus undergoes no further divisions. The 



B. 



FIG. 257. Pilularia Americana. A } cross-section of young sporocarp, showing four 

 sori, s ; f.b, vascular bundles (X 85). B, wall of ripe sporocarp (X 255). 



upper cell rapidly divides, and the single archegonium is soon complete. It has 

 a very short neck, and the neck canal-cell does not divide further, but otherwise 

 it is like the typical Fern archegonium. The spermatozoids collect in great 

 numbers about the macrospores, and sometimes completely choke the funnel- 

 shaped space in the mucilage above the open archegonium. 



In case fertilization is prevented, the prothallial tissue may continue to grow 

 for some time, and develops chlorophyll, which, in Pilularia, may be formed in 

 the absence of light. 



It has recently been shown that occasionally the embryo may develop without 

 fertilization one of the very few certain cases of parthenogenesis in the higher 

 plants. 



