124 HISTOLOGY OF ANGIOSPERM STEM, ROOT, ETC. 



Iii a foliage-leaf note the ridges on the inner (upper) 

 side of the leaf ; each ridge contains a bundle, and the 

 green parenchyma, with stomata, occupies the lower part 

 of each ridge, lining the furrows. A similar structure 

 may be seen in the leaves of various other G-ramineae. 



The leaves of Marram Grass (Psamma) and of some 

 moorland Grasses (Festuca, Aira, Nardus, etc.) are of 

 special interest from being rolled up ; in Psamma the leaf 

 can become partially unrolled in moist air, becoming closely 

 rolled up again in dry air. 



156. Xerophilous Leaf Structures. In various xero- 

 philous plants the leaves show characteristic structural 

 adaptations for the reduction of transpiration or the 

 storage of water. 



Among adaptations for checking transpiration note the 

 following: thick cuticle, often stratified ; sunken stomata, 

 lying below the general level of the leaf and often over- 

 arched by the surrounding epidermal cells ; development 

 of colourless " aqueous tissue " for water- storage, and of 

 hypodermis, above or below the mesophyll or in both 

 positions ; dense hairy covering ; waxy coating 011 the 

 cuticle; rolling-up of the leaf with the stoma-bearing 

 surface (upper surface in Grasses, lower surface in most 

 other plants) placed internally. In many cases, of course, 

 a xerophilous leaf shows several of these features. 



For (1) wax layers, in the form of grains or rods on the sur- 

 face, examine the leaves of Iris, Echeveria, Eucalyptus, etc. ; for 



(2) thick cuticle, Holly, Agave, Aloe, India-rubber Plant ; for 



(3) sunken stomata, India-rubber Plant, Oleander (stomata sunk 

 in groups in chamber-like infoldings of the lower surface) ; for 



(4) aqueous tissue, India-rubber Plant, Begonia, Peperomia ; for 



(5) hairy covering", Woolly Mullein, Hippophae ; for (6) rolled- 

 up leaves, Erica, Calluna, Empetrum, Nardus, Psamma, etc. 



157. Aquatic Leaf-structures. Examine and com- 

 pare the structure of (1) floating leaves, like those of 

 Pondweed (Potamogeton natans) ; (2) finely-divided sub- 

 merged leaves, like those of Water Crowfoot (often also 

 with floating leaves), Water Milfoil, etc. 



