THE EFFECT OF SLOPE EXPOSURE. 215 



In contrast with the other results tabulated, these are greater 

 on the southern side than on the northern. This somewhat 

 unexpected result is probably due to the fact already mentioned 

 that the method does not measure the actual transpiration, but 

 the maximum transpiring power of the leaf at the time. If this 

 is so, the trees on the northern slope have a lower transpiring 

 power, i.e., a greater power of resisting water loss, than do those 

 on the southern slope. 



SujniARY AND Conclusions. 



In the accompanying figure (p. 216) the series of observations 

 recorded in the foregoing tables are presented in graphical form. 

 The curves thus obtained show very clearly what has already been 

 repeatedly emphasised, that the differences between the two 

 slopes are comparatively slight during the morning, but become 

 very pronounced from noon onwards. Thus the curves for 

 sunlight intensity are very similar up to 11 a.m., after which 

 they begin to diverge, until at 2 p.m. they are widely separated. 

 A very noticeable feature of these curves is the sudden decrease 

 in the svmlight intensity on the southern slope after 2 p.m., the 

 intensity between 2 p.m. and 3 p.m. being even less than that on 

 the northern slope between 3.30 p.m. and 4.30 p.m. 



Both air temperature and rate of evaporation continue to 

 increase after the sunlight intensity has reached its maximum, 

 and the curves for these two factors appear to follow one another 

 fairly -closely, air temperature and rate of evaporation on each 

 side reaching a maximum at about the same time. The soil 

 temperature curves show striking divergences, that for the south 

 slope showing little rise and fall and reaching a maximum at 

 about 3 p.m., while that for the north slope rises steeply until 

 1.30 p.m., then falls slowly until 4.30 p.m., after which a more 

 rapid fall begins. 



The transpiration curves are interesting in that they appear 

 to follow the curves for sunlight intensity more closely than those 

 for air temperature or evaporation. This is particularly shown by 

 the curve for the southern slope, which shows a large decrease in 

 the period from 1.45 p.m. to 3.45 p.m. corresponding with the 

 similar decrease in sunlight which occurs after 2 p.m. This 

 suggests that the variations in the transpiring power are princi- 

 pally due to stomatal movements which are closely correlated 

 with variations in the light intensity, but the matter is one which 

 requires further investigation. 



The results of this preliminai-y investigation may be briefly 

 summarised as follows : — 



1. Marked differences exist in the character of the vegetation 

 on the northern and southern slopes of Signal Hill. 



2. These differences are probably principally due to differences 

 in the climatic conditions on the two sides of the hill and partly 

 to differences in the soil conditions. 



