THE ECOLOGY OF PLANTS. 



445 



of light, so that the only plants which can develop at the top 

 of the hedge-bank are either plants with long erect stems, 

 such as the Hedge-mustard, Nettle or Thistle, or climbing 

 plants such as Convolvulus, Hop, Clematis, Goose-grass. 

 Lower down the hedge-bank, where there is more moisture 

 and light, a great variety of plants may develop, while in the 

 shady ditch beneath we find various aquatic, amphibious, and 

 marsh plants. 



The hedge itself most commonly consists of Hawthorn, 

 but often of Gorse, Bramble, Sloe, or of small trees like 

 Willow or Hazel. These have usually been planted, as have, 

 of course, the hedges consisting of shrubs like Laurels, 

 Privets, Rhododendron, etc., around parks and plantations. 

 Why do hedges need to be pruned or " cut back," and what 

 is the effect of this operation? Notice that plants like 

 Dead-nettles, Yellow Toadflax, Campions, Stitchwort, when 

 growing close to the hawthorn (or other shrub forming the 

 hedge) push up between the branches and have long weak 

 stems, although the same species, when growing some dis- 

 tance from the hedge, has short erect stems with closely 

 arranged leaves. Cases like these suggest the way in which 

 plants, originally erect and self-supporting, may have become 

 climbers through lack of light and the presence of a suitable 

 support. 



It is very interesting to compare the nature and general 

 appearance or "habit" of the plants growing along the 

 north and south sides of a hedge which runs east and west. 

 The south side is, of course, warmer, drier, and receives 

 more light, and various plants grow on this side that 

 either do not grow at all on the north side, or are found 

 there in smaller numbers, and showing less luxuriant 

 growth. In the same way the trunk of a tree which is 

 growing in an open situation (i.e. not among others in a 

 damp wood) generally has its moister northern side covered 

 with a green powdery Alga (Pleurococcus, which is so 

 common on trunks and palings) and with various Mosses, 

 Liverworts, and Lichens, while the drier south side is 

 usually quite bare. In a damp wood the trunks and 

 branches of the trees are often thickly covered with these 

 " epiphytic " plants, often also bearing Ferns (especially 

 Polypody). 



