344 ECOLOGY 



disturbed ground become covered by plants. The first to 

 invade and colonize the ground are annuals with good 

 seed-dispersal mechanisms ; these are in turn succeeded 

 by perennials, many of which have effective means of vege- 

 tative propagation. The effect of shade and of exposure 

 on plant-distribution is obvious when we note how numerous 

 are the species on the sunny side, and how few on the shady 

 side, of a hedgerow. Similar differences may be seen in 

 hedgerows on opposite sides of a road running east and 



west. 



Walls 



In some districts, walls become so overgrown that, when 

 seen from a distance, they resemble hedgerows. This is 

 often the case in areas covered with glacial debris and where 

 stones of various shapes and sizes are abundant in the soil. 

 When the land is brought under cultivation, the stones are 

 removed and used for building walls, often of great thickness. 

 The interstices are filled with small stones and earth, form- 

 ing a soil which is very porous, and from it the water soon 

 drains away. Seeds which are deposited in the crannies 

 by the wind or animals may germinate, but as the plants 

 grow they are liable to suffer periodically from drought. 

 It is usual to find, therefore, that plants which succeed in 

 such a habitat are provided with devices to reduce tran- 

 spiration ; the leaves are either very small or arranged in 

 rosettes, and often modified as water-storage organs. Many 

 of the species are familiar as rockery plants, e. g. fleshy- 

 leaved forms like the Stonecrops, Saxifrages, and Wall 

 Pennywort (Fig. 221) . Heaths, Hair-grass, Gorse, and Broom 

 have reduced and wiry leaves. The Ivy-leaved Toadflax 

 has slender trailing stems and fleshy leaves ; the stalks of 

 its ripening fruits bend away from the light, lengthen, and 

 carry the capsules into the crannies, where the seeds are 

 shed. The Ferns found growing on the wall are all hardy 

 kinds, e. g. Wall-rue, Black Spleenwort, and Polypody. 



