i6 HOW TO KNOW THE MOSSES 



other instance, two different mosses were soaked and 

 dried fifty times without injury to the plants except 

 that the color of the leaves faded. 



A midrib, called a costa, may or may not be present, 

 and it may be single, double, or forked. In some of the 

 largest leaves it can be seen if the plants are held to the 

 light. (See Mnium punctatum var. elatum, Plate VI, 

 Fig. 5.) It shows like a dark line through the center 

 of the leaf, and is usually slender, but in the Polytri- 

 chaceae (p. 156) it is much wider, sometimes occupying 

 the greater part of the leaf blade. 



The leaves never have a stalk, but grow directly on 

 the stem and branch. 



Leaves on the same plant may vary in size. Those 

 that are typical and that should be selected for study 

 are along the middle part of the stem or branch. The 

 leaves near the base of the stem, as well as those at the 

 tip, are often smaller or not well developed. Plants not 

 bearing fruit sometimes grow more luxuriantly than 

 fruited specimens and therefore have larger leaves. 

 When the plants are much branched (pleurocarpous 

 mosses), the leaves on the stem are often larger than 

 those on the branches. Many leaves are large enough 

 to be seen easily, the longest measuring fully one-half 

 inch; many that are smaller can be seen when the 

 plants are held to the Hght or against something white; 

 while still others are too small or too closely folded 

 against the stem or branch to be seen without a lens. 



Leaves vary in shape from hair-like to nearly round, 



