TYPES OP CRYPTOGAMS; BRYOPHYTES 281 



its allies consist chiefly of the thallus in the vegetative con- 

 dition,- while the greater number of Hepaticse have a stem 

 and leaves. Thus they approach closely to the mosses. 

 But mosses usually have leaves on all sides of the stem, 

 while the leaves of Hepaticse are two-ranked, spreading 

 laterally, with sometimes a third row of leaves or scales 

 underneath. The leaves of mosses usually have more than 

 one layer of cells in some part, hut the leaves of the leafy 

 Hepaticse have but one layer of cells throughout. The 

 forms of the leaves are often very curious and interesting. 

 The sporophyte of most mosses consists of a capsule with 

 a lid, while in the leafy Hepaticse the capsule usually 

 opens by splitting longitudinally into two to four valves. 



Different species of Hepaticse grow on damp soil, rocks, 

 and the bark of trees. Many are capable of enduring 

 drought and reviving with moisture. 



THE STUDY OF PIGEON-WHEAT MOSS 

 (POLYTRICHUM COMMUNE) 



343. Occurrence. — This moss is widely distributed over the sur- 

 face of the earth, and some of its relatives are among the best 

 known mosses of the northern United States. Here it grows 

 commonly in dry pastures or on hillsides, not usually in densely 

 shaded situations. 



344. Form, Size, and General Characters. — Study several speci- 

 mens which have been pulled up with root-hairs. Note the size, 

 general form, color, and texture of all the parts of the plants exam- 

 ined. Some of them probably bear spore-capsules or sporophytes like 

 those shown in Fig. 206, while others are without .them. Sketch one 

 plant of each kind, about natural size. 



What difference is noticeable between the appearance of the 

 leaves in those plants which have spore-capsules and those which 

 have none ? Why is this ? 



