179 



armor, which may be of some use in reducing transpiration. 

 Many other species also have hairy leaves or stems. In fact, 

 such a protective covering is so common that the landscape 

 has not the ordinary green color, but is distinctly gray in 

 general tone. 



5. Involute or Conduplicate Lea res. — This adaptation is 

 shown by many of the grasses. 



6. Position of th<' Lcarcx. — In Helianthus occidehtalis the 

 thick, rough basal leaves stand with their blades nearly vertical. 

 Prostrate plants, like Mollugo verticillata and Euphorbia Geyeri, 

 may also be protected against excessive transpiration by their 

 position. 



As the surface of the sand dries out quickly after rains, 

 the first few inches contain very little water, and the roots of 

 the plants must penetrate through this dry upper layer into the 

 moist sand below. Many species accordingly have straight 

 tap-roots, which give off few lateral branches or none at all for 

 the first six to twelve inches. Some of the plants of this habit 

 are Polygonum tenue, Cycloloma atriplicifolium (PI. XIX., Fig. 1), 

 Froelichia campestris, Cristatella Jamesii, Croton glandulosus, 

 Euphorbia Geyeri, and (Enothera rhombipetala. Others have 

 thickened roots or rootstocks which serve for water storage, 

 Talinum rugospermum, Ceanothus americanus, and ]j)oin<r<i pan- 

 durata being examples. 



The sand-binding habit is best developed on the prairies, 

 where the action of the wind is most vigorous. Chief among 

 the sand-binders are the bunch-grasses (PL XIV., Fig. 2; XVII 1., 

 Fig. 1), which protect the sand from the wind by their dense 

 tufts of culms and basal leaves, and at the same time bind it 

 with their rootstocks and fibrous roots. If the bunches are far 

 apart, so that the sand is exposed between them, they frequent- 

 ly become raised several inches above the general level, hav- 

 ing held the sand beneath them while that not so protected 

 has been blown away. Species without sand-binding adapta- 

 tions are usually associated with the bunch-grasses and pro- 

 tected bv them. This condition is also found in the bunch- 



