2] THE VARIOUS GROUPS OF PLANTS 5I 



The spores, given suitable conditions after liberation, can germinate 

 to form fresh gametophytes, so completing the life-cycle. They 

 usually afford the main means of multiplication, although vegetative 

 methods, such as fragmentation of the gametophyte or formation by 

 it of special bud-like bodies called gemmae, may also be effective in 

 some cases. Liverworts chiefly inhabit damp places, such as 

 stream-banks, sheltered nooks, and the boles of trees and decaying 

 fallen branches in shady forests. They also grow on moist soil and 

 in tufts of Mosses, etc., and are practically world-wide in distribution 

 on land. Many grow on the leaves of other plants in the tropics, 

 and some in freshwater habitats. With the exception of a very few 

 which are saprophytic, the nutrition of the gametophyte is primarily 

 by photosynthesis ; on this the sporophyte is, as VvC have seen, in 

 most instances parasitically dependent. Although they are some- 

 times important as pioneers on bare ground and may even form 

 more or less ' pure ' patches some yards in extent. Liverworts in 

 general play only a minor role as ' fillers ' in higher vegetation. 

 Nor have they any particular value for Man, except sometimes as 

 aides in the binding and consolidation of eroding surfaces. 



Musci : These are the Mosses, which, in accordance with their 

 near relationship, are in many ways closely comparable with Liver- 

 worts. The Mosses are all rather small plants in which the gameto- 

 phyte, during the greater part (but by no means all) of its life, 

 consists of a more or less upright stem bearing small leaves. These 

 leaves, unlike their counterparts in leafy Liverworts, usually have 

 midribs and are spirally arranged on the stem, which may vary 

 from a fraction of an inch to perhaps a foot in length. The midribs 

 contain elongated cells, and a central strand in the stem usually 

 contains similar elongated cells which are supposed to conduct water 

 and nutrients. True roots are absent, but the base of the stem in 

 most types is plentifullv supplied with anchoring rhizoids. In one 

 characteristic and important group, known as Bog-mosses or Peat- 

 mosses, the leaf is not only peculiar in lacking a midrib but unique 

 in consisting of a network of small living cells separating large dead 

 ones which are transparent and perforated, soaking up and holding 

 water with extraordinary efficiency — hence the water-retaining 

 capacity of many bogs which are largely formed by such plants. 



On the gametophyte are borne the minute male and female organs, 

 commonly in groups that are made evident by the modification of 

 the surrounding leaves, and either on the same (hermaphrodite) 



