102 



HAIRS 



ing hairs are usually dead and, at maturity, occupied only by 

 air ; such dead hairs often appear white, showing that a large 

 proportion of the hght falling upon them is totally reflected, 

 hence they also afford protection against the heating effect of 

 the sun and against excessive illumination. Hairs thus serve 

 to reduce transpiration and act as a screen to the underlying 



chloroplasts. An 

 analogy could be 

 drawn with the 

 interior of a wood- 

 land, where be- 

 neath the trees 

 the air is cool, 

 shady, and damp, 

 just as it is be- 

 neath the hairs on 

 a leaf. Not un- 

 commonly hairs 

 are only present 

 on the young leaf, 

 falling off as the 

 latter matures [e.g. 

 1^1 an e. Horse 

 Chestnut, etc.). 



The simplest 

 type of covering 

 hair is unbranched 

 and usually tapers 

 towards the tip 

 (Fig. 48, B-D). 

 Short stiff hairs 

 of this kind are 

 found in some Boraginacere [e.g. I"orget-me-not, etc.), but 

 they are often much longer, and cither interwoven to form 

 a woolly tangle [e.g. Coltsfoot, Thistles, young leaves of the 

 Horse Chestnut, etc.) or all disposed in the same direction, giving 

 a silky appearance to the surface of the leaf [e.g. Silverwced). 

 Long unbranched hairs from the seed-coat of the Cotton-plant 

 [Gossypium spp.), a member of the Mallow-family (Malvacea:-), 



Fig. 40. — Hairs. A, of Deittzia; B, of Cynoglos- 

 sum ; C, of Shepherd's Purse (Capsella) 

 (unbranched type) ; D, of Vegetable Marrow 

 [Cucurbiia) ; E, of Hop [Humulus). 



