310 INTRODUCTION TO PLANT GEOGRAPHY [CHAP. 



Repeated burnings usually change the dominants and entire com- 

 munities to ones which can regenerate after fires and, so to speak, 

 withstand them. A few species, for example among the Grasses, 

 seem actually to be stimulated by exposure to fire, while some 

 Conifers, besides possessing thick and resistant bark, have cones 

 which are opened by fire to liberate the ripe seeds earlier than would 

 otherwise be the case. 



Although it may leave survivors open to fungous and other attack 

 through unhealed burn-scars, fire tends to favour the more resistant 

 species by removing their less resistant competitors ; it also alters 

 many factors of the environment that favour some species rather 

 than others. These principles apply to most types of vegetation, 

 including forests, heaths, and grasslands. And whereas the general 

 tendency of fire is towards vegetational degradation, the result is of 

 course influenced by climate and human agency quite apart from 

 the frequency and intensity of the burning. Indeed there are some 

 circumstances in which firing will benefit desirable species and 

 consequently may be intentionally practised by Man, as in pastures 

 in many parts of the world where the old growth is burned off 

 regularly. 



In conclusion we should outline the principles of cultivation, 

 through which Man now largely controls the plant life of much of 

 the land surface of the world and of some shallow waters. Cultiva- 

 tion consists basically in preparing the surfaces of suitable substrata 

 for the reception of seed (using this term in the widest sense), 

 normally after the area has been cleared of any natural vegetation. 

 Accordingly the early stages of growth of the crop are favoured by 

 a minimum of competition, although this may in time develop with 

 weeds or between the plants of the crop. Consequently effective 

 cultivation will include proper control of weeds and also sowing at 

 such intervals of space as will reduce the ill-effect of competition 

 to a minimum. Effort may also have to be given to maintaining 

 other conditions favouring the growth of the crop. 



In order to continue satisfactory cropping year after year in the 

 same area, it is commonlv necessary to repeat for each crop such 

 a ' tillage ' operation as digging or ploughing, which turns over the 

 soil and helps to maintain it in a suitable state. Lime or peat or 

 clay may also be added to some soils to improve their texture. 

 Often there has to be also some form of manuring, from animal or 

 mineral or chemical sources, to maintain the fertility of the soil by 

 replacing the necessary substances which are removed in cropping. 



