57 2 



ECOLOGY 



bearing surface (figs. 8n, 812). In xerophytic and hydrophylic dicotyls and gen- 

 erally in monocotyls, the side walls of the epidermis usually are straight on both 

 leaf surfaces (fig. 804), though in Maranta and in various grasses there are wavy 

 walls of striking regularity. In plastic species, waviness culminates in mesophytic 

 conditions ; increased and decreased transpiration each result in relatively straight 



lateral walls. Whether wavy 

 walls have a role of impor- 

 tance is not known, though 

 they have been thought to 

 add to the strength of the 

 epidermis and also to give a 

 greater diffusion surface for 

 substances passing from cell 

 to cell. 



813 



814 



FIGS. 813, 814. Appressed unicellular epidermal 

 hairs from a scale leaf of the winter bud of the Norway 

 maple (Acer platanoides): 813, a general view, as seen in 

 longitudinal section ; note the common orientation of the 

 hairs, which is responsible for the silky aspect of the 

 scale leaf; considerably magnified; 814, a single hair; 

 highly magnified. 



Structural features of 

 epidermal hairs. " Pro- 

 tective' 1 hairs commonly 

 are stiff, thick-walled 

 structures, which often 



are dead and air-containing at maturity. They may be attenuated uni- 

 cellular structures perpendicular to the leaf surface (as in Verbena or 



in Potentilla, fig. 914) ; more 



rarely they are parallel to the 



leaf surface and closely ap- 



pressed, their common orien- 

 tation giving the leaf a silky 



aspect (as in Aster sericeus 



and in the bud scales of Acer 



platanoides, figs. 813, 814). 



Other hairs are similar but 



multicellular, occasionally 



being branched (as in the 



mullein, fig. 815). A woolly 



felt, made up of a dense 



tangle of long hairs more or 



less parallel to the surface, 



extends in various directions 



(as in the everlastings and 



cinerarias, figs. 816, 817). 



Stellate hairs divide at the base into horizontal branches, as in various 



crucifers and mallows (fig. 773). In Shepherdia and in Elaeagnus 



FIG. 815. Branched multicellular hairs from 

 a leaf of the mullein ( Verbascum Thapsus) ; con- 

 siderably magnified. 



