

i9"] STARR— ANATOMY OF DUNE PLANTS 303 



bundles 



said, to differences in activity of transpiration. J 



mass 



even if 



transpiration was reduced to the minimum, and says "transpira- 

 tion can indeed influence the quality and quantity of the vessels > 

 but is not the cause of their formation. If it were so, the stems of 



thickness 



summer 



Haber- 



landt (18) relates the number and size of the ducts to the transpir- 

 ing leaf surface. Hartig (19) agrees with Haberlandt and adds 

 'in the damp air of a dense forest the inner spaces are much nar- 

 rower than in an open stand." Pfeffer (24) considers that within 

 certain limits the development of the conducting system is favored 

 when an increased demand is made upon it. Volkens (28), in 

 studies of desert plants, found a small development of water- 

 conducting elements. Cannon (5) irrigated desert plants and 

 compared their ducts with those of non-irrigated plants and found 

 better development in the latter. The two results may not be 

 inconsistent. Volkens' plants may have reduced leaf surface or 

 developed succulency, thus reducing transpiration, and so in a 

 way correspond with Cannon's irrigated plants. 



C onclusions 



Conditions in the dunes are severe for plant life, including direct 

 illumination and reflection, extremes of temperature, strong 

 winds, sand-blast, and sandy soil, the result of all these factors 

 being increased evaporation. The presence of considerable water 

 above the water-table makes conditions less severe than they 

 otherwise would be. The response to these conditions by true 

 dune plants is seen in the predominance of low vegetation, long 

 roots, woody stems, thick leaves (which may be reduced, equilat- 

 eral, evergreen, or folded), succulency, hairs, thickened epidermis 

 and cuticle, deep palisade, sunken stomata, and well developed 

 mechanical and conductive tissues in all parts. 



Plants generally growing in mesophytic situations, when found 

 also on the dunes, show the following modifications: of the leaf, 

 increased thickness, decrease in depth and increase in surface- 



