i 9 4 



STRUCTURE AND LIFE HISTORIES 



ness from four cells in the main stem, to one or two cells 

 in the smaller branches. The leaves are only one cell thick, 

 and are densely crowded on the stem, having, at maturity, 

 what is known as the two-fifths arrangement; that is, 

 if one starts with a given leaf and follows upward in a 

 spiral around the stem, he will pass five leaves before he 

 comes to one vertically above that with which he started, 



FIG. 139. Sphagnum sp. Upper portions of leafy plants, showing 

 sporogonia. 



and in doing this he will have passed twice around the 

 stem. The leaves of sphagnum never have a mid-rib 

 or other veins, and correlated with this is the entire 

 absence of any fibro-vascular bundles in the stem. This 

 is one of the features that marks the plant as of lower 

 organization than the fern. The stem forms numerous 

 branches, usually one for every fourth leaf, and glandular 

 hairs are usually met with at the bases of very young 

 leaves (Figs. 140 and 141). 



