554 MOSSES AND FERNS CHAP. 



Bruchmann (i) assert. Scott (2), however, states that in /. 

 hystrix, there is a short, cauline stele distinct from the leaf 

 traces. 



This central cylinder is composed of very short tracheids, 

 with spiral and reticulate markings, mixed with similarly- 

 shaped cells with thin walls. Surrounding this xylem cylinder 

 is a layer of cells, which Farmer calls the "prismatic layer." 

 This, according to Russow ((i), p. 139), is continuous with 

 the phloem of the leaf-traces, and he regards it as the phloem of 

 the stem bundle. Outside of this prismatic layer is a zone of 

 meristematic cells, which form the "cambium/' The cells of 

 this zone are like those of the cambium of Boytrychium or of 

 the Spermatophytes, and like these new cells are formed on both 

 sides; but those formed upon the outside remain parenchyma- 

 tous and are gradually thrown off with the dead outer cortex. 

 Those upon the inner side develop into the prismatic cells, 

 mingled with which are cells very like the tracheids, except 

 that they retain to some 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 thick- 

 ening of the stem occurs in Isoetcs, it is quite different from 

 that in Botrychium, which closely resembles the normal thicken- 

 ing of the coniferous or dicotyledonous stem. It has been com- 

 pared to that found in Yucca or Dracccna, and this perhaps is 

 more nearly like it. However, as the development of cambium 

 and secondary thickening have evidently occurred independ- 

 ently in very widely separated groups of plants, it is quite likely 

 that we have here orie 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 somewhat better 

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

 tracheids fully developed. The vascular bundle of the leaf is 

 better developed at the base of the leaf, and especially behind 

 the sporangium (Smith (i)). 



The phloem remains undifferentiated, and no perfect sieve- 

 tubes can be detected. The phloem lies upon the outer side of 

 the xylem, but show r s a tendency to extend round toward the 

 upper side. Of the Filicineae, Ophioglossum comes the nearest 

 to it in the structure of the bundles. The air-channels are four 



