292 MOSSES AND FERNS chap. 



extent their protoplasmic contents. These cells are arranged 

 in more >or less well-marked zones, and possibly mark the 

 limits of each year's growth. It will be seen from what has 

 been stated that while a true secondary thickening of the stem 

 occurs in Isoetes, it is quite different from that in Botrychium, 

 which closely resembles the normal thickening of the coniferous 

 or dicotyledonous stem. It has been compared to that found 

 in Yucca or DraccEna, and this perhaps is more nearly like it. 

 However, as the development of cambium and secondary 

 thickening have evidently occurred independently in very widely 

 separated groups of plants, it is quite likely that we have here 

 one more instance quite unconnected with the same phenomenon 

 elsewhere. 



The leaves, as already stated, differ but little from those of 

 the young plant. The vascular bundle is here somewhat better 

 developed, but remains very simple, with only a few rows of 

 tracheids fully developed. The phloem remains undifferentiated, 

 and no perfect sieve-tubes can be detected. The phloem lies 

 upon the outer side of the xylem, but shows a tendency to 

 extend round toward the upper side. Of the other Filicineae, 

 OpJiioglossuvi comes the nearest to it in the structure of the 

 bundles. The air-channels are four in number in the fully- 

 developed leaf, and the diaphragms across them more regular 

 and complete. Instead of being throughout but one cell thick, 

 as in the first leaves, they are thicker at the edges, so that in 

 section they appear biconcave. In the older leaves the broad 

 sheath at the base is much better developed, and the over- 

 lapping leaf bases give the whole stem much the appearance of 

 the scaly bulb of many Monocotyledons. In all the terrestrial 

 species, and those that are but partially immersed, the leaves 

 are provided with numerous stomata of the ordinary form ; 

 but in some of the submersed species these are partially or 

 entirely wanting. The development of the ligule also varies, 

 being very much greater in the terrestrial species, where it 

 may possibly be an organ of protection for the younger 

 leaves. 



Hofmeister-^ states that in /. lacustris the first sporangia 

 are not developed until the fourth year from the time the young 

 sporophyte is first formed. The sporophylls begin to form in 

 the third year, but it is a year more before the sporangia are 



^ Hofmeister (i), p. 364. 



