1 62 MOSSES AND FERNS chap. 



into an upper and a lower one, the former being the mother 

 cell of the leaf, the lower, with the other basal cell, giving rise 

 to the cortex. Growth proceeds actively in the young leaf, 

 which soon projects beyond the surface of the stem, and by the 

 formation of cell walls perpendicular to its surface forms a 

 laminar projection. The position of the cell walls in the young 

 leaf is such that at a very early period a two-sided apical cell 

 is established, which continues to function for a long time, and 

 to whose regular growth the symmetrical rhomboidal form of 

 the cells of the young leaf is largely due (Fig. 78). The leaves 

 do not retain their original three -ranked arrangement, but 

 from the first extend more than one-third of the circumference 

 of the stem, so that their bases overlap, and the leaves become 

 very crowded, and the two-fifth arrangement is established. The 

 degree to which the central tissue of the stem is developed 

 varies with the thickness of the branch. In the main stem it 

 is larg-e, but in the small terminal branches it is much less 

 developed, as well as the cortex, which in these small branches 

 is but one cell thick. Later the cortex of the large branches 

 becomes two-layered (Fig. yy, B), and is clearly separated from 

 the central tissue, whose cells in longitudinal section are very 

 much larger. In such sections through the base of very young 

 leaves characteristic glandular hairs are met with. They 

 consist of a short basal cell and an enlarged terminal cell 

 containing a densely granular matter, which from its behaviour 

 with stains seems to be mucilaginous. The form of the secret- 

 ing cell is elongated oval (Fig. yy, D), and the hair is inserted 

 close to the base of the leaf, upon its inner surface. 



The young leaf consists of perfectly uniform cells of a nearly 

 rhomboidal form (Fig. 78, A), and this continues until the apical 

 growth ceases. Then there begins to appear the separation 

 into the chlorophyll-bearing and hyaline cells of the mature 

 leaf. This can be easily followed in the young leaf, where its 

 base is still composed of similar cells, but where toward the 

 apex the two sorts of cells become gradually differentiated. 

 The future hyaline cells grow almost equally in length and 

 breadth, although the longitudinal growth somewhat exceeds 

 the lateral. These alternate regularly with the green cells, 

 which grow almost exclusively in length, and form a network 

 with rhomboidal meshes, whose interstices are occupied by the 

 hyaline cells. The latter at first contain chlorophyll, which 



