162 



STOMATAL GROOVES AND HOLLOWS 



thickness of the leaf. The stomata (St.) are confined to the 

 portions of the epidermis within these depressions, and inter- 

 spersed among them are numerous thick-walled unicellular hairs. 

 Each stoma is raised, on a papilla-like ring of cells, above the 

 level of the epidermis lining the hollows, a fact which is not 

 surprising when it is realised that, since the depressions contain 

 a damp atmosphere, the stomata within them develop under 

 the same conditions as those of ordinary leaves growing in moist 

 situations. Other striking features of the Oleander-leaf, apart 

 from the thick cuticle on the exposed surface, are the extremely 



lacunar spongy 

 tissue, and the 

 presence of 

 two layers of 

 water - storing 

 hypoderm be- 

 tween the 

 epidermis and 

 the palisade 

 tissue. 



In the She- 

 Oaks (Casua- 

 rina) of Aus- 

 tralia, which 

 supply valu- 

 able timbers, 

 the leaves are 



scale-like and carbon dioxide assimilation is carried on by the 

 green twigs, which possess longitudinal grooves (Fig. 83) to 

 which the stomata (S.), protected by hairs, are restricted. The 

 stomata are located at the sides of each furrow, and are 

 situated close to the assimilatory tissue, which is developed in 

 the same position as in other green stems (cf. below, p. 167). 



In the leaves of the Heather (Calluna, Fig. 84, B) the stomata 

 are confined to a single groove (stomatal chamber, St.) situated 

 on the under-surface of the leaf. The upper or "outer surface 

 is protected by a thick cuticle, and the aperture of the groove 

 is closed by interlacing hairs (Fig. 84, B). The form of the 

 transverse section is roughly that of an inverted triangle, with 



FIG. 83. Transverse section of a young assimilating 

 stem of the She-Oak (Casuarina) with the 

 stomatal grooves ; one of the latter is shown 

 enlarged on the right, ch., assimilatory tissue ; 

 5., stoma ; Scl., sclerenchyma. 



