CHAPTER XIX 

 LIVERWORTS AND MOSSES 



THE plants so far considered all agree in possessing relatively 

 simple sexual organs, consisting of more or -less modified cells, 

 whose protoplasmic contents, with or without division, give rise 

 to the gametes. The sexual organs, moreover, are usually, except 

 in such forms as Fucus and Pelvetia, distributed over the entire 

 thallus. On the other hand, in Liverworts (Hepaticae) and Mosses 

 (Musci), which are grouped as Bryophyta, the second class of the 

 Vegetable Kingdom, the sexual organs are not only much more 

 elaborate, but are commonly restricted to definite portions of 

 the plant-body. The latter usually also exhibits greater com- 

 plexity of structure, which may be related to the fact that the 

 Bryophyta are on the whole terrestrial plants in contrast, for 

 instance, to the essentially aquatic Algae. Other distinctive 

 features of the Bryophyta will become apparent in the subsequent 

 pages. 



The majority of Liverworts are damp-loving plants, many 

 of them growing in situations that are covered with moisture 

 in the wetter seasons of the year (e.g. along the sides of water- 

 courses), whilst some are even truly aquatic. Many Mosses, on 

 the other hand, can flourish in habitats that are comparatively 

 dry for a great part of the year, extreme examples being furnished 

 by the Hair Moss (Polytrichum juniperinum), found on dry 

 heaths, and the Wall Moss (Tortula muralis, Fig. 150, D), common 

 on old walls, rocks, etc. In relation to this difference of 

 habitat, Mosses generally display a structure which is more 

 elaborate, and better suited to resist drought, than that of 

 Liverworts. 



The body of the Liverwort in its simplest form is a small flat 

 green ribbon-like structure, often repeatedly forked, and growing 



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