376 ECOLOGY 



Horsetail, and Marsh Club-rush. Encroaching on these 

 invaders were : Water Plantain, Common Reed, Bur-reed, 

 Soft Rush, Marsh Horsetail, Marsh Bedstraw, Bog Stitch- 

 wort, Ragged Robin, Meadow-sweet, Hairy Willow-herb, 

 Square-stalked Willow-herb, Lesser Spearwort, Great 

 Valerian, Large-flowered Bitter-cress, Lady's Smock, 

 Marsh Marigold, Brooklime, Water Forget-me-not, Marsh 

 Thistle, Hemp Agrimony, Marsh St. John's-wort, Tufted 

 Hair-grass. Numerous trees surround the marsh, but only 

 two of the species have invaded it, the Alder and Willow. 

 These thrive well in the wet soil, and the marsh is 

 becoming converted into an Alder- Willow wood. 



CHAPTER XXXII 



WEEDS 



A careful examination of a natural piece of vegetation 

 produces the impression of a natural blend of colour and 

 form, and both are in keeping with the habitat. Such an 

 area contrasts sharply with a piece of cultivated land, which, 

 whether garden or farm, shows an obvious selection and 

 arrangement of plants suited to the needs or caprice of man. 

 Even here nature cannot be ignored, and it impresses itself 

 by topography, soil, and climate in a manner which 

 compels even cultivation to keep along more or less definite 

 lines. Nevertheless, man's aim is to substitute for the 

 less useful native plants those needed by him. In this way 

 much of the native vegetation is destroyed, but an interest- 

 ing mesh work persists along the hedgerows (Fig. 244), 

 roadside patches, and in ditches and streams ; or some 



