1 84 THE BIOLOGY OF FLOWERING PLANTS 



causal connection between the hairiness and the humidity, 

 or light rather hopeless to trace. In other plants, however, 

 Yapp found it easier to modify hairiness ; in Epilohium 

 hirsutum and Mentha aquatica hairiness was completely 

 suppressed by growth in a damp atmosphere. These 

 plants and others such as Lycopus europceus, Scabiosa 

 succisa, and Lysimachia vulgaris showed under natural 

 conditions the same general succession of leaf type as 

 Spiraea. 



We are evidently dealing here with another special case 

 of the very general succession of adult to youth forms. 

 The adult form is the hairy leaf, and it is related to normal 

 adult conditions ; in so far as it shows a diminution of 

 hairiness in poor light and higher humidity, it is exhibiting 

 a reversion to the juvenile form rather than a modification 

 in any one feature in response to special circumstances. 

 In some cases the regular course of development is very 

 definitely rigid, as in Spiraea ; in others, as with Mentha, it 

 is influenced markedly by the impact of the external complex 

 of conditions. 



Hairiness is, then, a very general xerophytic character ; 

 it may, in a single species, show marked exaggeration in 

 more arid stations, or at periods of development in which 

 the plant is exposed to more arid conditions. Thus the 

 American desert composite, Encelia farinosa, was found by 

 E. B. Shreve (1920), to form large glabrous leaves in the 

 wet, and small hairy leaves in the dry, season. Hairiness 

 is marked in many plants, as is indicated by the frequent 

 occurrence of such words as tomentosum as specific or 

 varietal names ; it may be assumed, although direct proof 

 is scanty, that it reduces transpiration and reflects light. 



§ 25. Effect of Ethereal Oils 



In certain dry regions the occurrence of plants with a 

 strong aromatic scent is very characteristic ; the scent of the 

 macchia of Corsica can be perceived far out at sea. The 

 lavenders, rosemary, rue, thymes, balms, lemon, oleander, 



