36 



BRITISH MOSSES. 



highly magnified the structure of the component cells in a 

 young and old leaf, and the grains of chlorophyll in the 

 cells. In the old leaf a tendency will be observed in these 

 grains to place themselves along the walls of the cells so 

 as to produce the effect of thickened walls. 



The leaves of Mosses stand in immediate connection 

 with the atmosphere, absorbing moisture from it when 

 moist, and shrinking and shrivelling when the air is dry. 

 In some cases they are characterized by a marked difference 

 in the form of the cells in the different parts of the leaf, 

 and again in other cases by the unequal distribution of 

 chlorophyll ; in other cases we come across 

 strange forms , the like of which we hardly 

 know in the phanerogams ; such are the 

 thick border and double rows of teeth in 

 some of the genus Mnium, the parallel 

 plates in Polytrichums and, stranger still, 

 the third flange of the leaf in Fissiden ; 

 the true homology of which has proved a 

 crux to bryologists. 



A drawing of the leaf of Fissidens 

 adiantioides is shown hi Fig. 21 a 

 thickened line of cells down the middle of 

 the leaf assumes very much the appear- 

 ance of a midrib, and on the right hand 

 side occupying the lower half of the leaf 

 is seen a third flange to the leaf, attached 

 at its upper part to the leaf in an oblique 

 line and after that to the vein or midrib 

 of the leaf, so that in that part of the 



Fi&. 21. Leaf of 

 Fissidens adian- 

 tioides, showing 

 a a, the third 

 flange. After 

 Schimper. 



