176 FORMATIONS AND GUILDS [Part II 



temperature is worthless in geographical botany, unless at least the amplitude of 

 the daily variation is also given. Mean annual extremes are important, not 

 indeed for the study of formations, but sometimes for the range of species ; the 

 mean annual temperature is quite unimportant. 



Data regarding the absolute rainfall are very useful, but not sufficient when 

 taken alone. Much depends on the fact whether the rain falls in relatively rare 

 but heavy showers, or in frequent and light ones. Of this we are informed 

 under the headings 'Rainfall in Da} r s ' ('Rainfall in Hours' would also be useful, but 

 this is hardly ever given) and ' Hours of Sunshine.' The last are also among the 

 factors influencing transpiration. 



Among the most important headings is ' Relative Humidity.' High atmospheric 

 humidity promotes growth and depresses transpiration ; low humidity acts in the 

 opposite direction. 



To winds great importance is attached owing to their desiccating action. The 

 heading ' Evaporation ' facilitates direct conclusions as to the strength of trans- 

 piration. 



3. THE EDAPHIC FORMATIONS. 



i. EDAPHIC INFLUENCES IN GENERAL. 



The influence of differences in the physical and chemical nature of 

 the soil is generally confined, as has been previously stated, to the 

 finer differentiations within the type of vegetation and of flora which 

 the climate determines. This edaphic differentiation is often extremely 

 rich, as many species correspond so exactly to a constellation of external 

 factors, that even slight changes in the latter remove the plants from 

 their oecological optimum and consequently cause their downfall in the 

 struggle with their competitors. 



Suppose for instance that we consider a meadow having a somewhat 

 uneven surface. Many of the predominant species especially among the 

 grasses are present everywhere, so that such a meadow may be described 

 as a single formation. Other species, on the contrary, appear confined 

 to quite definite kinds of soil, so that the meadow presents a chequered 

 appearance. This is especially the case where two or more allied species 

 have to divide the soil between them. If, for instance, Primula officinalis 

 and P. elatior are present, we can from a distance distinguish by means 

 of the dissimilar tints of the flowers the drier spots occupied by the 

 former, from the damper spots where the latter grows. We shall never 

 find them growing side by side. In a similar way, Ranunculus bulbosus, 

 R. acris, and R. repens frequently denote three grades of increasing 

 humidity. On the Simplon, two species of dwarf Senecio, S. incanus 

 and S. uniflorus, frequent the dry alpine meadows, often close together, 

 but never intermixed. I found the large-headed Senecio uniflorus only 

 in places where the meadow-soil extended in a thin coating over stones 



