534 MOSSES AND FERNS chap. 



ficiently obvious. It is, however, interesting to note that Sel- 

 aginella seems to have retained certain characters that are ap- 

 parently primitive. These are the presence of a definite apical 

 cell in the stem and root of most species, and the peculiar chlo- 

 roplasts, which are especially interesting as a possible survival 

 of the type found in so many Confervaceae, e. g., Coleochcete, 

 from which it is quite likely that the whole archegoniate series 

 has descended. This form of chloroplast occurs elsewhere 

 among the Archegoniatse only in the Anthocerotes. 



In the characters of the sporangium and the early develop- 

 ment of the prothallium, Selaginella undoubtedly shows the 

 closest affinity to the Spermatophytes, especially the Gymno- 

 sperms, of any Pteridophyte. The strobiloid arrangement of 

 the sporophylls and the position of the sporangia are directly 

 comparable to the strobilus of the Coniferse. The wall of the 

 sporangium is here not only morphologically, but physiologic- 

 ally comparable to the nucellus of the ovule, and the macro- 

 spore grows, not at the expense of the disorganised spo- 

 rogenous cells and tapetum alone, but is nourished directly 

 from the sporophyte through the agency of the cells of the 

 sporangium stalk and wall, until the development of the en- 

 closed prothallium is far advanced. The latter, both in its 

 development while still within the sporangium, as well as in 

 all the details of its formation, shows a close resemblance to 

 the corresponding stages in certain Conifers. The formation 

 of a "primary" and "secondary" prothallium is, as we have 

 seen, only apparent, and the diaphragm in the prothallium of 

 Selaginella is not a true cell wall, marking a primary division 

 of the spore contents, but only a secondary thickening of the 

 lower walls of certain cells, indicating a temporary cessation in 

 the process of cell-formation. It is by no means improbable 

 that this cell-formation may sometimes go on uninterruptedly, 

 in which case no diaphragm would be formed, and, as in Isoetes, 

 there would be no distinct line of demarcation between the 

 archegonial tissue at the apex and the large-celled nutritive 

 tissue below. 



The presence of a suspensor in all investigated Lycopodinese 

 is a character which distinguishes them at once from the other 

 Pteridophytes, and has its closest analogy again among the 

 Conifers. 



The possibility that the Psilotacese may not be directly re- 



