183 



return for food in the shape of fruit, etc., while some ants in 

 return for food provided by the plant protect the latter against 

 other injurious insects. 



The following sub-divisions of the subject will now be con- 

 sidered in detail : 



Influences of other Plants on plant development. 



Influences of the Soil on plant development. 



Influences of the Atmosphere on plant development. 



Effect of fire on plant development. 



SECTION II. INFLUENCES OF OTHER PLANTS ON PLANT DE- 



VELOFMENT. 



161. Following the classification 



proposed above, we will consider, in order, the effect exercised 

 by plants in their capacity as (a) Competitors, (b) Parasites, 

 and (c) Symbionts. 



(a) Competitors, 



162. We have to consider here 



not only the competition among roots for the water and Plant Com- 

 mineral salts in the soil, but also that among shoots f or P etlt rs - 

 the sun-light and air. In so far as competition with the 

 higher plants above ground goes, their insignificant de- 

 velopment in the air renders the Cryptogams of little or 

 no importance, but in the soil their competition may be 

 injurious. The mycelia of many saprophytic fungi, for 

 example, are known to compete actively with the root hairs of 

 higher plants in the soil by taking up large quantities of valuable 

 mineral salts, especially of phosphorus and potassium. The 

 most injurious competitors with the higher plants, however, are 

 undoubtedly to be found in the ranks of the higher plants 

 themselves, i.e. among the Phanerogams. This subject has 

 already been alluded to in Part IV above, see p. 148, and only a 

 few striking examples of competition can be mentioned here. 



In the mixed forest of Deodar and Blue Pine in the Deodar and 

 Western Himalayas, the latter has continually to be girdled Blue Pme 

 to prevent its suppressing and ousting the more valuable 

 Deodar. A similar state of things exists in all our Indian 

 mixed forests and the forester has to girdle, or fell, trees which, 

 as yet, are of no value to prevent their vanquishing the better 

 kinds in this competition for the necessaries of life. 

 Another well-known case of competition is afforded by the Teak and 

 relations existing between Teak and various gregarious species 

 of bamboos. In Burma, for instance, the dense shade afforded 

 by the gregarious Bambusa poiymorpha often effectually 



