572 



ECOLOGY 



bearing surface (figs. 8ii, 812). In xerophylic and hydrophytic dicolyls 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. 



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" 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). 



Fig. 815. — Branched multicellular hairs from 

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

 siderably magnified. 



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

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



