STOMATAL GROOVES AND HOLLOWS i6i 



the cuticle being relatively thin and the stomata often slightly 

 raised above the general surface (Fig. 81, C, D). 1 



Retarded transpiration is also very effectively attained by 

 restriction of the stontata to grooves or hollows whose commu- 

 nication with the exterior is often partially occluded by an 

 outgrowth of hairs. A good example is furnished by the common 

 Oleander (Nerium oleander). If the lower surface of a leaf of 



FIG. 82. Transverse section of part of the leaf of the Oleander (Nerium 

 oleander) showing a stomatal chamber. Col., collecting cells ; 

 e , epidermis ; p., palisade layers ; St., stoma ; V, vascular bundle. 



this plant be examined with a lens, a large number of light- 

 coloured patches appear dotted between the principal veins, 

 each patch being due to a tuft of hairs arising from one of the 

 numerous hollows. In transverse sections (Fig. 82) the latter 

 are seen to extend inwards for slightly more than one-third the 



1 Plugging of the stomatal apertures with particles of wax is observed 

 in some plants (e.g. certain Conifers), which thereby impede the escape of 

 water-vapour and consequently check transpiration, which is almost 

 entirely cuticular. 

 71 



