lo] ENVIRONMENTAL FACTORS 289 



feature most important for trees, the season in which it falls may- 

 matter a great deal to herbaceous plants and grasslands. These 

 last are especially favoured by spring rainfall in regions of cold 

 winters. With hot and dry summers but winters warm enough 

 for growth, there may be a preponderance of sclerophyllous shrubs 

 [i.e. having rather small leathery leaves). With less and less rainfall 

 there tend to be still more xeromorphic plants {i.e. with features 

 aiding them to conserve water, as illustrated in Fig. 20, xerophytes 

 being in general plants which grow in dry places). At the other 

 extreme are hydrophytes, living more or less submerged in water 

 (Fig. 21), and hygrophytes, or marsh plants, while between xerophytes 

 and hydrophytes lie the so-called mesophytes, living in habitats that 

 usually show neither an excess nor a deficiency of water. 



Rain is caused by the cooling of moisture-laden air. Rainfall is 

 usually reported in the form of monthly means, which are the 

 amounts falling in the various calendar months but averaged over a 

 period of years, though the number of rainy days in each month 

 gives a better indication of its distribution. Moreover, sudden 

 heavy rain is apt to be largely lost as run-off, and may cause bad 

 erosion. Because of the often marked local differences with physio- 

 graphic changes, it is desirable for an ecologist to have his own 

 automatic rain-gauge which, like his thermograph, needs tending 

 only once a week. 



Other forms of precipitation are snow (which may lie on the 

 ground to form a valuable protective blanket and also a reservoir 

 of water, but is apt to limit the growing-season by its late melting), 

 hail (which may cause serious injur}^ especially to young crops), 

 sleet, and dew (which is important in some deserts where it pro- 

 vides much of the surface water on which the ephemeral plants 

 depend). Fig. 81 indicates the average annual precipitation in 

 different parts of the world. 



(4) Evaporating power. The evaporating power of the air is a 

 factor of the utmost importance to the life of plants, as it directly 

 affects their transpiration. It is indicated approximately by the 

 relative humidity (the ratio of the water-vapour present in the 

 atmosphere to that necessary for saturation at a particular tempera- 

 ture), or, more accurately, by the ' saturation- deficit ', which takes 

 account also of temperature, and determines the ' pull ' exerted by 

 the atmosphere on the water-economy of plants. 



The relative humidity is commonly measured by means of a 

 ' wet and dry bulb ' hygrometer (psychrometer), the difference in 



