892 BOTANICAL SURVEY OF DISMAL SWAMP REGION. 



is less pervious to heat rays than is ordinary atmosphere. I low effect- 

 ive this may be is yet very doubtful, 1 but it is not to be denied lhat such 

 aromatic plaids are much more abundant in dry soils and climates 

 where the water supply of the plant needs to be jealously guarded than 

 where oilier conditions prevail. On the dunes the principal species 

 thus characterized are Myrica carolinensis, Iva inibricata, Chenopo- 

 dium anthelminticum, and Monar da punctata. 



(7). Formation of a sand envelope around the roots. About the 

 root fibers of certain grasses (notably Aristida spp.) of the North 

 African deserts, Volkens 2 observed the presence of a cylinder com- 

 posed of sand grains that cohere with each other and adhere closely to 

 the root hairs by means of a viscous excretion from the latter. In 

 the deserts of Arizona and in other arid sandy wastes the same 

 phenomenon occurs. This Volkens believes to be a protection against 

 excessive loss of water in its passage through the axial vascular strand 

 of the root. In Ammoplula arenaria and Uniola panic id a ta^ on the 

 Virginia coast, it was observed that sand grains cling tightly to the 

 persistent root hairs, often adhering even when the plant is uprooted 

 and roughly handled, although no viscous excretion was detected. 

 Some protection may thus be afforded the plants in question against! 

 loss of water in its course through long roots, although the adaptation 

 is obviously much less perfect than in grasses of the deserts. 



(8) Development of tunicated bases of the stems (especially in Gra- 

 mineae). The persistence of the bases of old sheaths at the foot of the 

 culms in Gramineae, as pointed out by Ilackel, is a character espe- 

 cially noticeable in grasses of arid regions. It is believed to afford 

 protection against transpiration from the base of the culm, which rests 

 in the frequently much-heated surface sand. The enveloping sheaths, 

 therefore, perform the same function as do the homologous dry outer 

 scales of many bulbs, and are said to be further useful in some cases 

 as a reservoir for water, which is held between the closely appressed 

 sheaths. In I hi tola panic.id.ata, Ammophila arenaria, and Panwwm 

 amarum, among dune grasses, the tunic sheaths may serve the former 

 purpose, but do not form a dense enough covering to be useful for 

 storage of water, even were there necessity for such a structure in 

 strand plants of this moist climate. 



The strand plants of this region generally differ from xerophytic 

 formations elsewhere in their lack of those specialized structures 

 which enable the plant to reserve water against a period of drought." 



'Pfeffer (Pnanzcnphys. 2te Auflage 1:501) considers this absorption " as hardly 

 of high importance" for protection against loss by water. 



'■' Flora der agypt.-arab. Waste, p. 25. 



:, E.g., the water hairs of speciesof Mesembryanthemum in the Sahara (Volkens, 

 Flora der agypt.-arab. Waste, p. 53) and the cup-like leaf bases of species of 

 Tillandsia and other tropical epiphytes (Schimper, Die epiphytische Vegetation 

 Auierikas, pp. 7'd, 74, etc., 1888). 



