244 BOTANY 



studied. The spermatozoid retains its original form for some time, 

 and is recognizable even after it has penetrated into the cavity of 

 the egg-nucleus. Here it slowly changes form, approaching the con- 

 dition of the original sperm-nucleus. The chromosomes become 

 more evident, and finally are not distinguishable from those of the 

 egg-nucleus. 



The egg-cell has, in the meantime, been slowly growing, and is 

 provided with an evident cell-membrane. The first division may 

 occur after an hour or so, as in species of Marsilia ; but in the com- 

 mon Ferns it is probably at least a week in most cases, and in other 

 Ferns still longer. 



The Embryo 



The embryo in its earlier stages (Fig. 209) closely resembles that 

 of the Bryophytes, and in the lower types of Pteridophytes the 

 development of the special organs, leaf, stem, and root, may not 

 occur until the embryo has reached a relatively large size. Sooner 



or later, however, by 

 the development of a 

 B J\^})\ leaf an d root, the young 

 sporophyte becomes in- 

 dependent. 



When the young sporo- 

 phyte is established, the 

 gametophyte dies, and 

 leaves the sporophyte as 

 an independent leafy 

 FIG. 209. A, Onoclea sensibilis. B, Riccia glauca. plant, which in most 

 Embryos, showing the similarity in the embryo cases lives many years, 

 of the Ferns and Liverworts in the early stages , ~ 



(x200). Sooner or later the 



sporophytic character of 



the plant is shown by the production of spores, which resemble in 

 all respects those of the Bryophytes. They are borne in special 

 organs, Sporangia, which are of very characteristic structure in the 

 different groups of Pteridophytes. In the simplest case, that of 

 Ophioglosstim, they are cavities in the leaf tissue filled with a great 

 number of spores. Usually (Fig. 210) they are capsules, often 

 stalked, borne upon special leaves, sporophylls, which are sometimes 

 quite different from the foliage leaves, and arranged in a spike or 

 cone, suggesting the flowers of the simplest Seed-plants. 



Spore-division 



The sporogenous tissue may sometimes be traced back to a single 

 archesporial cell, but this is not always the case. The sporogenous 



