284 INTRODUCTION TO PLANT GEOGRAPHY [CHAP. 



functional responses or differences in growth or structure, although 

 different plants vary widely, of course, in the nature and degree of 

 such response. 



Climatic 



The climatic factors comprise the general features of regiona 

 climate, often being rhythmic — exhibiting, for example, diurnal, 

 seasonal, or long-term cyclic fluctuations. They may also vary 

 locally, to give local climates, and even do so in extremely restricted 

 confines, to give microclimates. Examples of local climates are 

 found on steep northern or southern hill-slopes, and of microclimates 

 on the leeside of boulders which protect the immediately neighbour- 

 ing plants and animals from wind and insolation. In general the 

 factors classed as climatic have a dominating influence. Neverthe- 

 less, so far as plants are concerned, these factors are often but 

 poorlv expressed by meteorological records which, for example, are 

 usually taken at some standard height above the ground (rarely the 

 height of the plant) and fail to observe the often rapid fluctuations 

 which can be so important to a sensitive organism. There are five 

 main climatic factors which we must now consider in turn: it is 

 their different combinations which largely prescribe vegetation-types, 

 and so they cannot well be placed in order of relative importance. 



(i) Light. This, as we have seen, is essential for photosynthesis, 

 though fortunately sufficient illumination for this purpose is present 

 almost everywhere on land and in surface waters ; it may also be 

 important for some reproductive processes. Fig. 79 shows the 

 Moss Campion [Silene acaulis agg.) flowering only on the top and 

 south-facing side of its domed tussocks in high-arctic Spitsbergen, 

 where it can constitute a reliable compass ; the effect is thought to 

 result from differing light intensities. 



The light-climate at a spot depends on the duration, time- 

 distribution, intensity, and quality of the insolation there obtaining, 

 though so far as the plant is concerned the effective period may be 

 modified by cold or drought. Moreover, for successful flowering 

 many plants require a relatively long (or, alternatively, short) day, 

 apparently regardless of the intensity or quality of the light, and 

 consequently such plants are largely limited to high (or low) latitudes, 

 respectively. The effect of light on photosynthesis depends largely 

 on intensity, which also influences growth. In the open, elongation 

 is checked and lateral organs enlarge, whereas with congested 



