l6 CONNECTICUT GEOL. AND NAT. HIST. SURVEY. [BuU. 



important differences in this case being in the gametophytes. 



In most of the Metzgeriacese the gametophyte is a thallus, 

 but a few of the genera show a more or less complete differen- 

 tiation into stem and leaves. The plants are usually composed 

 of parenchyma throughout, but a few thalloid species develop 

 a very primitive conducting tissue composed of elongated cells 

 with lignified walls. The archegonia are borne on the upper 

 surface of the gametophyte or of a special branch, and do not 

 directly terminate its growth. In many cases a protective 

 structure is developed outside the calyptra, and this sometimes 

 assumes the form of a pseudoperianth as in the Marchantiacese. 



The Jungermanniacese are sometimes called Scale Mosses, 

 the gametophyte being invariably a leafy stem. Most of the 

 species are prostrate, and the plants show a distinct dorsi- 

 ventrality, even when ascending or erect. The leaves are 

 normally alternate and arranged in three ranks, two of which 

 are turned toward the light and the third toward the sub- 

 stratum. The leaves of this third rank are called underleaves, 

 and are usually much smaller than the others and different 

 from them in form. Sometimes they are so much reduced in 

 size that they can scarcely be demonstrated, and in a few 

 genera they are absent altogether. The two ranks of large 

 leaves usually spread out in such a way that the whole shoot 

 acquires a strongly flattened appearance, very characteristic 

 of the family as a whole. 



The leaves as a rule exhibit no cell differentiation whatever, 

 and are invariably destitute of midribs. They show, however, 

 a great deal of variation in form and in the way in which 

 they are attached to the stem. They are sometimes undivided, 

 sometimes variously toothed, lobed, or deeply cleft ; they are 

 sometimes developed in one plane, sometimes variously folded ; 

 they are sometimes attached by a continuous line, sometimes 

 by two lines which meet at an angle. In a few genera the 

 leaves develop peculiar organs, known as water sacs, in which 

 water may be temporarily retained. The branches sometimes 

 show a differentiation into those which bear normal leaves 

 and those which assume a flagelliform appearance, the leaves 

 in the latter case being strongly reduced or even absent 

 altogether. The flagelliform branches frequently perform the 



