398 ECOLOGY 



grasses and sedges (Fig. 255), or back-rolled as in heaths 

 (Fig. 253), with stomata sunk in pits or grooves. The 

 blades are usually small, leathery, hairy or fleshy, and often 

 arranged in compact rosettes, forming the cushions so 

 familiar in rockery plants like the Saxifrages (Fig. 257), 

 Houseleeks, Cushion Pinks (Silene acaulis), or the woolly 

 leaves and flowering shoots of Edelweiss (L. alpinum). 

 Some plants form dense mats of interlacing trailing stems. 

 Such are the procumbent Azalea, Crowberry (Fig. 253, 1), 

 Alpine Club Moss (Lycopoditim alpinum), the dwarf forms of 

 Bilberry, and Alpine Willows. These form a flat carpet 

 with their numerous rhizomes matted in the soil. Sections 

 of the Willow-stem may show many (from fifteen to 

 twenty) annual rings, but in spite of their age the plants 

 only grow a few inches from the ground. 



Insects are scarce, and usually of the lower types ; yet 

 many alpine plants produce large, showy flowers like the 

 Pinks, Saxifrages, and Gentians. Vegetative reproduction 

 by means of offsets, runners, and rhizomes is prevalent. 

 Viviparous plants (see p. 141) are not uncommon on the 

 mountains. Examples are Sheep's Fescue (Festuca ovina), 

 Alpine Poa {Poa alpina), Alpine Bistort, and some sedges. 



The study of plants and their distribution reveals the 

 great power of adaptation which they possess. In relation 

 to the factors of the environment, plant-organs are modified 

 in a great variety of ways and present remarkable contrasts 

 in form and structure, e. g. water-plants and plants of the 

 sea-coast or the Heather moor ; plants of the moist, shady 

 woodland and the dry, sunny desert ; trees of the lowland 

 forest and the flowery cushions of the mountains. Our know- 

 ledge of the conditions under which these varied forms grow, 

 leads us to conclude that the main features of the vegetation 

 of a country are determined by the conditions of the habitat. 



The plants of a given association, however, react on the 

 habitat, and, by changing the conditions of the habitat, 

 prepare the way for new forms and the extinction of the 



