i8o THE BIOLOGY OF FLOWERING PLANTS 



pit arrangement. The furrows of Casuarina bear hairs, and 

 the grooved lower surfaces of the leaves of Loiseleuria and 

 Erica are hairy. The production of hairs is a very wide- 

 spread occurrence, and ranges from sparse and short bristles, 

 or fringes along the leaf edge and about the veins (as in 

 the beech), to dense tomentose coverings which give the 

 leaves — of the Edelweiss, to take a famous example — a grey 

 or white appearance. The hairs are epidermal outgrowths 

 and show a wide range of form and structure ; they may 

 be unicellular or multicellular, simple or branched, straight 

 or wavy ; or they may assume such strange forms as the 

 scales of Elaeagnus, the oleaster, or the bifid prongs of the 

 wallflower. Conspicuous hairiness is a common feature of 

 xerophytic plants ; it is a mark of many of the plants of the 

 garigue and macchia of the Mediterranean regions, e.g. of 

 Cistus albidus, the lavenders, the rosemary, and others ; it 

 is prominent in many mountain plants growing in exposed 

 positions. When the hairiness reaches the tomentose stage, 

 that is when the hair forms a thick covering or felt, it is likely 

 that it acts in much the same way as the sinking of the 

 stomata in pits, by preserving a layer of moister air between 

 them and the external atmosphere. It is also held that it 

 keeps down the leaf temperature in the sun by reflecting and 

 absorbing light. A certain amount of evidence is available 

 to show that " shaved " leaves attain a higher temperature 

 than those with their natural covering. When the hairs 

 are sparse the effect must be very much smaller. Renner 

 (1909) holds that with sparse hairs the most effective type 

 of hair is that which lies close to the epidermal surface, 

 as in the wallflower. It is likely that such a covering may 

 tend to prevent movement of the surface layer of air. In 

 many plants where the degree of hairiness varies, it is marked 

 in dry stations, while in humid surroundings the leaf may 

 be almost glabrous. A striking example of this is afforded 

 by Polygonum amphihium, which is known in two distinct 

 habitat forms. As an aquatic it has floating leaves, oblong- 

 oval in form, with a polished upper surface, and quite 

 glabrous. On the land the leaves are smaller, narrower, 



