PROTECTED STOMATA 



179 



walled outgrowths of the neighbouring parenchyma, or by 

 masses of resinous material. 



The protection given by grooves is increased by the 

 very general tendency of such leaves to roll in edgewise in 

 drought. This is well seen in Psamma and Elymus, where 

 the leaf rolls completely into a narrow tube in dry weather. 

 Less pronounced cases are those where the two leaf halves 

 fold together as in Festuca glauca and Aira ccespitosa. 

 Examples are common in grasses of steppes or dry pastures. 

 In such cases the stomata occur in grooves on the upper 

 surface only, and it is this surface which comes to lie on 

 the inside of the rolled or folded leaf. 



The mechanism of this movement has not been quite 

 cleared up. Along the bottom of the furrows of grass leaves 



Fig. 20. — Leaf of Aira ccespitosa : transverse section showing the 

 deep furrows at the bottom of which are the thin-walled cells which are 

 concerned in the folding in of the leaf : mechanical tissue shaded. X 40. 



there lie larger thin- walled water-storing epidermal cells. 

 It is Hkely that these, as they lose water, have their walls 

 drawn together, and so cause a local contraction of the 

 upper surface. Whether the strong sclerenchyma, which 

 clothes the lower surface, takes an active part in the 

 movement or acts merely as a resistance is not known. 

 Haberlandt's account should be consulted (cp. Fig. 20). 



§ 24. Hairiness and Transpiration 



Further protection of the stomata may be gained where 

 the leaf is clad with hairs. This is often combined with the 



