CHAPTER XII 

 ECOLOGICAL RELATIONS 



THE growth of the plant . is influenced by many factors, 

 among the most important of which are water, soil, temperature 

 and light. The combination of these four factors may not be 

 such as to give the maximum in growth and productiveness ; 

 for while one or two of these factors may be in the correct pro- 

 portion for the best growth of the plant, the other two may be 

 in proportion unsuited for the greatest possible development. 

 The plant is also influenced by animals, wind currents, and 

 many other factors too numerous for discussion at this time. 

 When a factor is such as to kill or prevent the growth of the 

 plants of any particular species, it becomes a limiting factor 

 in the spread of the species regardless of the character of the 

 other factors. Lack of water may prevent the spread of a 

 species into a territory when all other factors may be satisfac- 

 tory. This is, well illustrated by the great desert regions of the 

 world which produce abundant vegetation when subjected to 

 irrigation. Any species of plants will extend its range as fast 

 as the controlling factors will permit, and will reach its max- 

 imum development where the combination; of the factors is best 

 suited to its existence and its minimum where it is most 

 unsuitable. 



Water. We have already learned that plants cannot live 

 without water and that it is the most abundant compound in most 

 plants and also one of the most important. It passes into the 

 plant by means of the root-hairs and passes out of the plant by 

 transpiration, mostly through the leaves. We have also learned 

 that many plants are composed almost entirely of water. How- 

 ever, plants differ in the amount of water required for their 

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