SPECIAL TYPES OF STOMATA HAIRS 101 



are so strongly thickened as to leave only a small slit-shaped 

 cavity (Fig. 46, B), but the end-portions are thin-walled and 

 somewhat enlarged (Fig. 46, A, g.c.). On either side of the 

 stoma is a thin-walled subsidiary cell (s.c.). These latter, to- 

 gether with the terminal portions of the guard-cells, function 

 in much the same way as the thin-walled part of an ordinary 

 guard-cell, whilst the thick median portions correspond to the 

 thickened walls of the latter. 



Submerged leaves of water-plants usually bear no stomata, 

 whilst in floating leaves they are restricted to the upper surface, 

 and exhibit marked differences from the ordinary type. The 

 guard-cells, in transverse section, are here roughly triangular 

 through the inner walls being bevelled off towards the outer 

 edge of the pore, where the thickening is most pronounced. 

 As the upper surface of such floating leaves is usually covered 

 by a fine waxy bloom (e.g. Water Lily, Nymphcza), this side of 

 the leaf is not readily wetted, and hence the formation of water- 

 films across the stomatal apertures is prevented. 



Not uncommonly a few, or even many, of the epidermal cells 

 (cf . Fig. 37) grow out into more or less elongated, often branched, 

 processes called hairs, which are especially common on leaves 

 and, when numerous, are very obvious to the naked eye. They 

 may remain unicellular (Fig. 48, B, C), or become multicellular 

 (Fig. 48, D) by the formation of septa whose development is 

 probably related to mechanical requirements. Nearly every type 

 of hair, whether branched or not, may be uni- or multi-cellular 

 without any appreciable difference in outward form; but the 

 presence or absence of septa, as well as the structure of the hair, 

 are often characteristic of whole groups of plants. The walls of 

 the hairs are of varying thickness, and either consist of cellulose 

 or have undergone chemical alteration. 



One of the most important functions of hairs, when numerous, 

 is to bring about a decrease in the rate of transpiration. The 

 moist air entangled between these hairs is sheltered from the 

 wind, so that it is not readily removed by air-currents, nor does 

 it diffuse rapidly into the dry atmosphere around. As a result 

 of the presence of this moisture-laden air in the immediate 

 neighbourhood of the leaf -surf ace, transpiration of water- vapour 

 from the interior through the stomata is retarded. These cover- 



