Leafy Mosses 



from the next line. If the two fractions are reduced to the same 

 denominators y T and f\ n T and compared, it will be seen that the 

 leaves were but little more crowded. The extra crowding is 

 compensated for by the greater distance between two succeeding 

 leaves in the same line and by the fact that the leaves in the 

 second instance are narrower than the first. 



HOW A HAIRY-CAP AVOIDS TOO STRONG LIGHT 



The devices for avoiding the extreme heat of the sun are per- 

 haps still more wonderful than those for obtaining a sufficient 

 amount. It is a fact that in the leaves of the Hairy-caps only the 

 upper surface of the leaves is so constructed as to be injured by 

 too dry heat. The cell walls of the lower surfaces are on the 

 contrary thick and impervious to water, so that they cannot give 



Bryum argenteum. Leaf 

 with open cell - structure 

 and midvein extending 

 only part way to the apex. 



Calharinea undulata. Cross 

 section of leaf to show the leaf- 

 blade one cell thick, and the 

 lamellae rising from a thickened 



vein. 



Dicramtm flagellare. Part 

 of leaf to show open cell- 

 structure of base. Solid vein 

 on the right of cut. 



up moisture to the air when it is dry, a character which insures 

 against loss by evaporation, for when the air is dry the mosses 

 simply turn the awn-pointed leaves upwards with the points and 

 the impervious under-surfaces to the sun and the delicate cells 

 toward the stem. 



VEGETATIVE PART 

 (Gametophyte) 



The structure of the leafy-mosses is mostly very simple. The 

 leaves are generally but one cell thick from surface to surface, 

 except along a line from apex to base where they form a 

 mid- vein (costa). 



43 



