THE PHYSICAL ENVIRONMENT OF ORGANISMS 555 



a relative humidity of 15 to 25 per cent (open sand-scrub and blackjack 

 oak habitats) to 85 to 95 per cent (below the shrubs and herbage of deep 

 woods and swamps). The lower humidities are far beyond the limits of 

 toleration for amphibians and many insects, and these animals show a 

 type of hydrotropism that keeps them within areas of relative humidity 

 as near as possible to their optima. Some of the lizards and insects of 

 this region, on the other hand, are well adapted against water loss and 

 move freely into or live only in the xeric sand-scrub areas. 



Humidity and temperature together. So far we have considered tem- 

 perature and humidity separately. If one wished to make the simplest 

 and clearest experiments concerning their effects on the behavior of 

 organisms, he would attempt to hold the one constant while he varied 

 the other in his experimental setups. But in nature conditions are gen- 

 erally not simple. When both temperature and humidity are varying, it is 

 often impossible to be certain which is the more stimulating or limiting 

 factor. Moreover, for a given organism the limits of toleration and the 

 optimum for temperature change with changes in humidity, and vice 

 versa. Man, for instance, at a humidity of 20 to 30 per cent, can tolerate 

 for several hours temperatures that would be quickly fatal at a humidity 

 of 80 per cent or more. Air-conditioning engineers have found that for 

 each temperature there is a certain optimum humidity within which 

 man is most comfortable and works with greatest efficiency. For each 

 temperature this optimum humidity lies at a different point on the scale 

 of percentage of saturation. 



Light. Aside from furnishing all the energy for photosynthesis, light 

 has many important effects on the reactions and behavior of organisms. 

 Many organisms have limits of toleration and optima for its various 

 intensities, and much of the behavior of both plants and animals can be 

 traced to positive or negative phototropisms to certain of these intensities. 

 The positive phototropism of moths, beetles, and flies to lights at night 

 is particularly interesting in view of the fact that nearly all these same 

 insects show a marked negative phototropism to all but the weakest 

 intensities of daylight. Practically all green plants show a marked positive 

 phototropism in the direction of growth of their green parts. This is often 

 strikingly conspicuous in plants grown indoors, and in the differences of 

 shape that develop in two trees of the same kind when one grows in a 

 forest and the other in an open field. 



The behavior of many organisms is also influenced by the proportion 

 of light and lightless hours in the day. This is most conspicuous in those 

 regions of the earth where marked differences in length of day are a part 

 of the change in the seasons. Changes from summer parthenogenesis to 

 autumnal bisexual reproduction in certain insects and the migratory 

 behavior of certain birds have been found to be largely regulated by 



