INTRODUCTION t 



tion from the leaves during insolation only about 60% to 75% of the 

 total incident energy is absorbed. Of this amount a small fraction, 

 say 1%, is used in photosynthesis and a still smaller amount is re- 

 leased by respiration. The evaporation of water from the internal 

 surfaces of the leaves, or transpiration uses up a large amount of 

 the energy absorbed. Yet transpiration is by no means an unmiti- 

 gated evil, simply dissipating energy. Heating large quantities 

 of water serves the plant in much the same way that it serves an 

 internal combustion engine, it reduces the temperature. With liv- 

 ing plants the process is carried on to the evaporation of water, but 

 without large increase in temperature. Brown and Escombe have 

 shown that the leaf exposed to full sunlight is receiving energy 

 fast enough to raise its temperature almost 30° C. per minute. At 

 this rate two minutes of strong sunshine would result in the coagu- 

 lation of protoplasm and the instant death of the plant. Clearly 

 then the role of transpiration is in keeping the plant alive. Though 

 the efficiency of vegetation may not be high, the balance is in favor 

 of absorption, and this small percent of energy assimilated and 

 stored by green plants is the foundation upon which all the super- 

 structure of organic life is built. 



The second, the edaphic factors include in a broad general way 

 the influence of soils upon plant growth. It is from the soil that 

 plants receive the minerals necessary for life in raw fonn. These 

 are built up into organic compounds in the plant and are assimilated 

 as food after being combined with the products of photosynthesis. 

 Water taken in by green plants also comes for the most part, if 

 not entirely, from the soil. Soil temperature and its oxygen supply- 

 ing power are still other factors important in the life of the plant. 

 The physical and chemical nature of the soil then is included in the 

 term edaphic. 



The last, the biotic factors, include the effects of plants and ani- 

 mals upon the development of the vegetation. The chestnut bark 

 fungus has already destroyed large forests of the Allegheny Moun- 

 tain region and the white pine blister rust is menacing not only 

 the white pine forests of the east but some of our most important 

 timber pines of the western part of North America. Leaf eating 

 insects often destroy enough foliage to kill or seriously weaken 

 green plants. It has been stated by Professor Herbert Osbom that 

 grass hoppers and leaf hoppers consume as much of the vegetation 

 in a pasture as the cattle. Sap sucking insects are often as destruc- 

 tive. Rodents and grazing animals alter the aspect of vegetation. 



