298 PHYSIOLOGICAL BOTANY. PART II. 



But from the latitude of Edinburgh northwards, the 

 difference increases in favour of the subsoil. The 

 consequence is that certain plants which naturally 

 belong to the more temperate parts of our zone, are 

 enabled to extend themselves further north and south 

 than they could do if the mean temperature of the soil 

 and air were every where the same. 



(30-t.) Influence of Light. The influence of light 

 is less essential than that of temperature in fixing the 

 geographical limits of different species, though it is cer- 

 tainly of great importance in many cases. Light is, as 

 we have seen, the chief agent in stimulating the vital 

 properties, and its effects are apparent in a great 

 number of vital phenomena, such as the absorption of 

 the sap, the exhalation of moisture, and the decom- 

 position of carbonic acid. It is probable that each 

 species requires a peculiar stimulus from different de- 

 grees of light as well as of heat, and we find that such 

 as are succulent, resinous, or oily, generally prefer situ- 

 ations where they can obtain most light ; whilst many 

 evergreens and others grow best where they are some- 

 what shaded. In these respects alpine plants may be 

 contrasted with maritime species, the former receiving 

 the greatest and the latter the least light, under the 

 same degree of latitude. Whilst the mean distribution 

 of light is more nearly equable for all latitudes than the 

 mean temperature, the variations in the mode of its dis- 

 tribution are much greater. Contrast, for instance, the 

 alternate long continuance of light and darkness at the 

 poles with their nearly equable daily distribution at the 

 equator. 



(305.) Influence of Moisture. The proportion in 

 which water is supplied, constitutes one of the chief 

 peculiarities of every " station ; " and plants are very 

 differently constituted with respect to the precise supply 

 which they require to preserve them in a healthy con- 

 dition. Those which require most, have a loose and 

 spongy texture, with large and soft leaves, and little or 

 no pubescence, but many stomata ; whilst such as grow 

 in arid districts are frequently firm and succulent, often 



