GROWTH REGULATION 279 



Biotic Factors 



As described earlier, bioiic factors are those which connote competi- 

 tive growth on the part of other plants antl animals in close proximity 

 to the one of immediate interest. Thus, living organisms surrounding 

 a particular plant form as much a part of its environment as do tem- 

 perature, light, mineral salts in the soil, and other climatic and 

 edaphic factors. In fact, biotic factors influence the growth behavior 

 of the plant throughout its life. As might be expected, some factors 

 favor plant growth whereas others are distinctly inhibitive to such 

 growth. 



Plants which are crowded close together are forced to compete with 

 each other for space, light, water, and mineral salts in the soil. Thus 

 there is competition both above and below ground. Such overcrowding 

 obviously results from failure to thin plants of the same variety or 

 failure to remove undesirable plants (weeds) of other varieties. 



Whereas many plants depend upon insects to effect pollination, 

 these same plants may be destroyed by the ravages of other insects, 

 bacteria, or fungi. For example, certain bacteria, fungi, nematode 

 worms, and insects live parasitically on the roots of jilants. Moreover, 

 many different kinds of insects, fungi, viruses, and bacteria attack the 

 aerial portions of a plant, causing destructive diseases. 



Soil fertility is also influenced by the activity of certain fungi and 

 bacteria which cause decomposition of organic matter into materials 

 available to green plants. Some soil bacteria also bring about nitrogen 

 fixation. Certain soil fungi influence the growth of some plants, 

 particularly forest trees. In this instance, the soil fungi attach them- 

 selves to the roots of the trees, sometimes even penetrating the living 

 root cells. The presence of such a fungus is quite often beneficial in 

 that the parasite makes readily available to the host some of the nitro- 

 genous compounds of the soil. Such fungi are also beneficial in that 

 they facilitate absorption and transfer of minerals from the soil to the 

 host. 



Finally, grazing animals contribute greatly to the alteration of 

 vegetative growth in a particular area. This effect may vary from 

 selective removal of certain plants (thus encouraging growth of others) 

 to complete overgrazing by certain animals with the ultimate destruc- 

 tion of rangcland. With regard to the latter, it is well known that 

 the perennial grasses of rangeland of our western states have been 

 seriously depleted as a result of excessive and improper grazing. 

 Rejuvenation of such grassland is a time-consuming and costly opera- 

 tion. 



