i82 THE BIOLOGY OF FLOWERING PLANTS 



in the upper leaves there are two layers of typical palisade, 

 and in the lower the palisade is much reduced ; sun leaf 

 character goes with hairiness, and vice versa. 



We can relate hairiness to arid conditions in two ways. 

 In the first place it is most pronounced in leaves unfolding 

 in summer ; this is most marked in the succession on adult 

 non-flowering shoots referred to in Yapp's summary. 

 Records taken with atmometers showed that the period of 

 maximum evaporation, June, July, and August, corresponds 

 exactly with the production of the hairiest shoots. In the 

 second place the position in which the hairy leaves are borne 

 on the stem is where evaporation is most rapid. This 

 brings us to speak of an important relation which has been 

 worked out for marsh vegetation by Yapp in another paper 

 (1909). In any plant community with fairly tall, close 

 vegetation, the conditions as regards evaporation at the 

 upper level of the plant shoots are very different from those 

 lower down, where a considerable degree of sheltering takes 

 place. Yapp investigated this by taking atmometer readings 

 4 ft. 6 in. above the soil (just clear of the vegetation 

 tops), 2 ft. 2 in. above the soil, and 5 in. above the 

 soil (at the bottom of the vegetation). During eight days 

 in July the evaporation in the three stations was in the ratio 

 100 : 37'6 : ici. During eight days in August and 

 September the ratio v/as 100 : 28 : 3*2. Temperature 

 records showed a marked lowering at ground level. The 

 main part of the difference in evaporation was probably 

 due to shelter from wind in the lower strata. It is obvious 

 that leaves near ground level are living in an entirely 

 different environment from those 4 ft. up as regards 

 humidity, wind action, temperature, and also light. This 

 difference in environment is reflected in the presence of 

 different layers of vegetation. Plants like Hydrocotyle, 

 Symphytum, and Scabiosa, and the radical leaves of Spiraea 

 inhabit the ground layer ; Carices, and the greater part of 

 the leaf surface of Angelica and Lysimachia occupy the 

 middle layer ; the upper leaves of these and of Spirsea form 

 the upper layer. This is usually referred to as a stratification 



