PLANT COMMUNITIES. 



80 



As the amount of transpiration is related in a general way to the 

 extent of leaf area on a plant, it is obvious that a reduction in the 

 amount of leaf surface will be accompanied by diminished transpiration. 

 Hence in xerophytic species, especially those with woody stems, there is 

 often a very scanty leaf-development, much of the assimilation being- 

 then performed by the young green twigs. The leaves may even lose 

 their assimilatory function entirely, becoming reduced to minute scales, 

 in which case their function is necessarily performed by the stems, as in 

 some species of Spartium, Asparagus, &c. 



In Pimis and other conifers, Ericacece, &c, the leaves are narrow 

 and needle-like, and in many Heaths and Grasses have inrolled margins. 

 In nearly all xerophytes the exposed surface of the leaf is covered by a 

 very thick cuticle or by a dense covering of hairs, both of which con- 

 trivances diminish loss of water ; while the stomata are usually few in 

 number and sunk below the surface or placed in the furrows in grooved 

 leaves, or confined to the under surface of rolled leaves, so as to be more 

 or less completely shut off from the outer air. 



In countries where wet and dry seasons alternate many plants spend 

 the dry period in a dormant state as underground bulbs, tubers, rhizomes 

 or thickened roots, and only vegetate on the return of the wet season. 

 Many trees and shrubs shed their leaves at the approach of the dry 

 season, just as do the trees of temperate climates on the approach of 

 winter. In some countries, notably in Australia, it is common for leaves 

 to be so arranged on the branches that their edges instead of their surfaces 

 are turned towards the sky and ground. The leaf surface thus escapes 

 direct exposure to the sun's rays except when it is near the horizon and 

 consequently least powerful, and so in this way also is the loss of water 

 minimised. 



Among xerophyte societies may be mentioned "rock societies" con- 

 sisting of Lichens and Mosses and other plants which can contrive to live 

 on exposed rock-surfaces ; " Heath societies " characterised by Heather, 

 Heaths and other Ericacece, Furze, Kest-harrow, Wild Thyme, Sedum acre, 

 Galium varum, and various Grasses ; the Pine forests of northern and. 

 mountainous regions; while the "bush" and "scrub" of Africa and 

 Australia, and the Mexican Cactus-deserts, represent the extreme of 

 xerophytic conditions. 



Many arctic and alpine plants show marked xerophytic features, such 

 as dwarfed and tufted habit, closely packed and inrolled leaves, thick 

 cuticle and sunken stomata, hairy surface, leathery or fleshy texture, &c. 

 Among these may be mentioned the dwarf Birch and Juniper, Cherleria, 

 Silctie acaulis, Draba, Saxifraga oppositifolia and other species, Azalea, 

 Vacoinium, Arctostapliylos and other Ericacece, Cerastium alpiuioii. 

 Edelweiss and its allies, Sedum Bhodiola, &c. 



Many epiphytes — plants which cling for support to other plants, but 

 derive no nutriment from them and are not rooted in the soil— show 

 well-marked xerophytic characters, and frequently develop organs for 

 collecting and storing water. Such are the epiphytic Orchids and Ferns. 



Mesophytes form the bulk of the familiar vegetation of temperate 

 inland regions, where the physical conditions show no very striking 

 extremes of temperature or dryness or moisture, where the soil is more 



