] 



■ 

 COMPOUND CONDITIONS AFFECTING PLANT GROWTH. 15 



the amount of heat, and lias usually been computed by adding together 

 the excesses 'of each day's mean over 32° F. for all the days of the 

 season. There is still another clement of heat which this method 

 does not take cognizance of, namely, midsummer extremes. Two regions 

 may have the same annual mean, summer mean, and summer amount 

 of heat, yet the customary occasional occurrence of frosts in one of them 

 may cause the absence of many species which occur in the other. No 

 one of these four measures of heat— mean annual temperature, mean 

 summer 1 temperature, amount of summer heat, and degree of sum- 

 mer extremes of temperature— expresses precisely the thermal sum of 

 a region as related to its plant life; yet any one of them is a general 

 guide to this sum, and an intelligent consideration of all four gives the 

 nearest approximation at present attainable. 



The effect of altitude is mainly reducible to that of the accompany- 

 ing temperatures. It has not been demonstrated that variation in the 

 pressure of the air has any appreciable effect upon plants. An increase 

 in soil moisture, however, usually accompanies in arid regions any 

 considerable increase in altitude. !t results not directly from the 

 greater altitude, but from the precipitation caused by the generally 

 lower temperature. In a humid climate this relation does not appear 



to hold. 



The intervention of oceanic bodies of water between continental 

 areas is of very great importance in limiting the spread of the accom- 

 panying floras. Not merely species, but generic types, are commonly 

 confined by this agency within a single continental area. The cause 

 may be reduced to unfavorable conditions of mechanique— the impos- 

 sibility of the transportation of seed. 



Proximity to large bodies of water, such as seas, lakes, and large 

 rivers, is often the cause of a northward extension of a more southern 

 flora, and is explained principally by the more uniform spring and fall 

 temperatures. 



Slope exposure is a condition whose effects are felt wherever the sur- 

 face of the ground has a steep slope. The south side of a mountain 

 the inclination of which is 20° from the horizontal, receives the sun's 

 rays much more directly than does either a level plateau or a north 

 slope, and therefore has a higher temperature. An illustration of this 

 effect is furnished by the desert mountains, in which the lower limit of 

 trees on a north slope is often a thousand feet lower than on a south 

 slope; and if a ravine with sloping sides occurs on the eastern expo- 

 sure of the mountain at the altitude of about this line, its south side, 

 sloping north, will be covered with trees, while the north side, sloping 

 south, will be naked. 



Differences in variation due to changes in soil are more conspicuous 

 to the average observer than those due to any other cause. No item 



i The term summer is here used for the entire yearly period of growth and repro- 

 duction. 



