230 



ANALYSIS OF THE ENVIRONMENT 



cides with the active radiating surface. 

 When such plants cast an effective shade, 

 the temperature at the ground level at noon 

 on a midsummer's day is lower than that 

 found 2 meters higher in open exposure to 

 sun and wind. At night the minimum tem- 

 perature is still located near the ground, 

 and the air becomes steadily cooler until 

 free equihbrium is reached well above the 

 vegetation. The relations are shown in 

 Figure 59. 



2° 

 NIGHT 



Fig. 59. Temperatures in a snapdragon flower 

 bed in August, in Munich. Height above 

 ground level in centimeters; higher tempera- 

 tures shown at the right. (Modified from 

 Geiger. ) 



The contrasting conditions found with 

 close stands of tall grass are illustrated in 

 Figure 60. The maximum absorption by the 

 surface of the stalks Hes below the upper 

 surface of the vegetation. Under many con- 

 ditions, radiation penetrates relatively far. 



Fig. 60. Temperatures in a stand of winter 

 rye in May, near Munich. Height above 

 ground in meters. The temperature curve bends 

 to the left to show lower temperature. (Modi- 

 fied from Geiger.) 



and in short grass or grains like wheat or 

 rye the temperature maximum is at ground 

 level. As the grasses grow taller, the level 

 of maximum temperature also moves up- 

 ward; the temperature falls in the upper 

 levels of grass or grain until it reaches ap- 

 proximate equilibrium with the air above. 



At night, in a rye field, the minimum tem- 

 perature is found some distance above the 

 ground. The air is cooled about the heads, 

 but as a result of the density of the lower 

 stalks and leaves, the chilled air can sink 

 only part way to the ground so that the 

 minimum lies about midway between the 

 upper surface of the grain (or grass) and 

 the earth. 



These examples were selected to show 

 that, without measurements, the height at 

 which maximum or minimum temperatures 

 occur in a given type of vegetation can be 

 only roughly approximated from general 

 rules known at present. The case illustrated 

 by night conditions among coarse vegetation 

 (Fig. 59) shows that a plant matrix of 

 this sort would provide no protection from 

 frost to insects living in it, since cold air 

 sinks through l)ie coarse stand all the way 

 to the ground. Dense stands of grasses 

 would supply protection from frost near 

 ground level. Both these types of vegeta- 

 tion, as well as forests, give considerable 

 protection from direct sunshine. 



The control of temperature and of other 

 physical factors by aggregation of both 

 poikilothermons and homoiothermous ani- 

 mals will be discussed under the heading of 

 aggregation (p. 398). The still more inti- 

 mate biotic modification of the temperature 

 in symbionts (including parasites) is of 

 evident importance. 



Wind 



The modification of wind by the plant 

 matrix is as obvious as are the temperature 

 relations. Wind velocitv is reduced by even 

 a fringe of trees. Witness the familiar 

 "windbreak" of prairie farmsteads. The 

 natural ribbons of timber along the streams 

 of the North American Great Plains con- 

 stitute effective natural windbreaks. Within 

 dense forest or even in dense growth of 

 shorter vegetation, air movement may be 

 reduced to convection and diffusion and 

 may approximate stagnation. 



In the rain forest of Panama, wind move- 

 ment reaches its highest level during the 

 drv months of January, February, and 



