HIPPURIS LEAF 139 



absent from both surfaces : discoid hairs are occasionally 

 seen. 



iii. The mesophyll, which is much less bulky, and is 

 reduced to a single layer of cells in the marginal parts, 

 is not differentiated into palisade and spongy parenchyma. 



iv. Only one central vascular bundle is present. 



The absence of stomata should be confirmed by tan- 

 gential sections from both upper and lower surfaces : at 

 the same time note the discoid hairs in surface view. 



A comparison, in respect of the distribution of stomata, 

 may be made with floating leaves, e.g. of Nymphcea or 

 Nuphar, in which stomata will be found on the upper 

 surface which is exposed to the air, but not on the 

 lower submerged surface. 



I. Multiple epidermis. In not a few leaves the epidermis 

 (derived from the single layer of dermatogen) consists of more 

 than a single layer of cells, owing to periclinal divisions of the 

 cells of the dermatogen : this multiple epidermis is sometimes 

 styled aqueous tissue. 



Cut sections of the lamina of various species of Begonia, and 

 note the narrow green band of mesophyll, bounded above and 

 below by bands of transparent aqueous tissue, the number of 

 layers varying in different species, and the upper bands being 

 broader. 



Similar observations should be made on leaves of species of 

 Peperomia, in which, as in Begonia, the upper band of aqueous 

 tissue is the broader, extending even to fifteen or sixteen layers 

 of cells in some species. 



Sections should also be cut of the lamina of Ficus elastica 

 (the India-rubber Plant), or other species : note the three rather 

 irregular layers of colourless tissue composing the upper epidermal 

 band, the outermost consisting of the smallest cells : also the large 

 cells with cystoliths : the narrower band of lower epidermis, con- 



