172 MOSSES AND FERNS CHAP. 



of the secreting cell is elongated oval (Fig. 89, D), and the 

 hair is inserted close to the base of the leaf, upon its inner sur- 

 face. 



The young leaf consists of perfectly uniform cells of a 

 nearly rhomboidal form (Fig. 90, A), and this continues until 

 the apical growth ceases. Then there begins to appear the sep- 

 aration 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 differ- 

 entiated. The future hyaline cells grow almost equally in 

 length and breadth, although the longitudinal growth some- 

 what 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 occu- 

 pied by the hyaline cells. The latter at first contain chloro- 

 phyll, which soon, however, disappears ; and finally, as is well 

 known, they lose their contents completely, and in most cases 

 round openings are formed in their walls. The protoplasm is 

 mainly used up in the formation of the spiral and ring-shaped 

 thickenings upon the inner surface of the wall, so characteristic 

 of these cells (Fig. 90, D). The chlorophyll cells are some- 

 times so crowded and overarched by the hyaline ones that they 

 are scarcely perceptible, and of course in such leaves the green 

 colour is very faint. Cross-sections of the leaves show a char- 

 acteristic beaded appearance, the large swollen hyaline cells 

 regularly alternating with the small wedge-shaped sections of 

 the green cells (Fig. 90, E). Russow (4) has shown that the 

 leaves of the sporogonial branch retain more or less their primi- 

 tive character, and the division into the two sorts of cells of the 

 normal leaves is much less marked. He connects this with the 

 necessity for greater assimilative activity in these leaves for 

 the support of the growing sporogonium. From his account 

 too it seems that the stem leaves lose their activity very early. 



The degree of development of the thickenings upon the 

 walls of the hyaline cells varies in different species, and in dif- 

 ferent parts of the leaf. It is, according to Russow, best de- 

 veloped in the upper half of the leaf, where these thickenings 

 have the form of thin ridges projecting far into the cell cavity. 



The development of the central tissue of the stem varies. 



