54 INTRODUCTION TO PLANT GEOGRAPHY [CHAP. 



Mosses grow on a wide variety of exposed surfaces — particularly, 

 but by no means entirely, in damp situations. Thus they occur 

 plentifully on the ground, on tree-trunks in moist woodlands, on 

 decaying wood, on old brick and stone structures, on rocks and 

 boulders, and in both still and running fresh water. They are also 

 common as subsidiary forms in higher vegetation. Mosses are 

 more numerous in species and individuals than Liverworts, and 

 tend to cover considerably larger areas and to be far more conspicuous 

 — especially in arctic and boreal regions, and high up on mountains. 

 They are relatively important as components of natural vegetation 

 and frequently dominate substantial areas especially of bogs, whose 

 water-level they often raise. In so doing they may even destroy 

 tracts of forest and make terrain difficult to traverse, thus affecting 

 the economy of Man. On the positive side they are important as 

 producers of peat, which often consists largely of the remains of 

 Bog-mosses, and as stabilizers of sand-dunes and other erosive 

 systems whose surfaces they help to bind. Peat is used extensively 

 as fuel and in the improvement of soils. Owing to their insulating 

 properties when dry. Bog-mosses are also used in construction work 

 and packaging, and, owing to their absorptive and water-retaining 

 powers, for surgical dressings and the transport of living plants. 



The above groups all belong to the non-vascular cryptogams and 

 lead up to the vascular plants {Vasculares, or Tracheophytes), to 

 which all the remaining groups belong. The vascular plants are 

 those possessing a vascular system, and include all the most advanced, 

 or evolutionarily ' higher ', types ; these are generally the largest 

 and most dominant on land. A vascular system consists mainly of 

 special tracts (bundles) of elongated wood (xylem) and ' bast ' 

 (phloem) cells forming a continuous system linking all the main 

 parts of the plant. Its chief manifestations are such bundles in the 

 stem and veins in the leaf. The primary purpose of such a vascular 

 system is the conduction of water, mineral salts, and elaborated food 

 materials to portions of the plant where they are needed, so that it 

 is partly comparable with the blood and lymphatic systems of higher 

 animals. Its secondary function is to give mechanical support, 

 especially in the * secondarily thickened ' older stems of perennial 

 plants which consist largely of vascular tissues and have the whole 

 crown to support. An account of the general make-up of a vascular 

 plant was given in Chapter I, with some indication of how ' The 

 Ideal Plant' lives and grows (pp. 12-14). 



