244 



MOSSES AND FERNS 



CHAP. 



the former probably being directly the initial fof the plerome 

 cylinder. The outer cell by later divisions forms the cortex, 

 and the epidermis which covers the very small exposed surface 

 of the stem apex. Here, as in Ophioglossum, it is impossible 

 to determine exactly the method of origin of the young leaves, 

 one of which probably corresponds to each segment of the 

 apical cell, but as soon as the leaf can be recognised as such 

 it is already a multicellular organ. It grows at first by an 

 apical cell which seems to correspond closely in its growth 

 with that of the stem. From almost the very first (Fig. i 24) 



Fig. TZ^.—Boirychiuvi Virginianum (Sw.). A, Longitudinal section of the stem apex of a young 

 plant, X 260 ; B, cross-section of a similar specimen ; L, the youngest leaf. 



the growth of the leaf is stronger on the outer side, and in 

 consequence it bends inward over the stem apex. 



The arrangement of the tissues of the fully -developed 

 stem shows, as we have seen, a striking similarity to those 

 in the stems of many Spermaphytes. The xylem of the 

 strictly collateral bundle is made up principally of large 

 prismatic tracheids (Fig. 125), whose walls are marked with 

 bordered pits not unlike those so characteristic of the Coni- 

 ferae, but somewhat intermediate between these and the 

 elongated ones found in most Ferns. The walls between 

 the pits are very much thickened, and the bottoms of 



