54 



group, but some, as Lesquerella argcntea, are placed among the in- 

 terstitials. 



The second group includes the mat-plants, a small group with but 

 three flowering plants, Opuntia Rafmesquii, Opuntia fragUis, and An- 

 tennaria sp. SclagincUa riipestris is also common in the Hanover 

 area. These grow close to the sand and tend to spread annually over 

 a larg-er area. They are unable to encroach upon either the bunch- 

 g-rasses or the perennials, and do not survive when covered by mem- 

 bers of these groups. Their number is accordingly largest in the 

 more open consocies. These plants are of the greatest importance in 

 binding sand, and under certain conditions have a prominent part 

 in stabilizing blowing sand. Sclagiiiclla riipestris is especially note- 

 worthy for its habit of circular growth. Extending vegetatively 

 from the center, its growth is so regular that a circular patch is 

 formed. This is soon converted into a ring by the death of the 

 center, leaving a marginal zone of living plants one to two inches 

 (2-5 cm.) wide. This ring gradually increases in size until it may 

 reach a maximum diameter of four feet (1.2 m.). Additional rings 

 may begin within an old one, or parts of two rings may overlap. 

 Megaspores are produced in enonnous quantities, but their successful 

 growth must be rare. The prickly pear, Opuntia Rafincsquii, is much 

 more common in the Havana area than in any of the others. The 

 mats of SclagincUa are favorite places for small mats of a dark col- 

 ored crustv species of Cladonia. Small mosses, of unidentified 

 species, are also freciuently found, and in many places a dark-colored 

 crust on the sand is formed by a species of Oscillatoria, which from 

 its habit may also be classified with the mats. 



The third group, the interstitials, is composed in general of an- 

 nuals, with slender, frecpiently unbranched stems, generally narrow 

 leaves, and fibrous roots. They come up late, principally during the 

 season of heavy rainfall in June or July, and cover the bare areas of 

 sand with prodigious numbers of individuals. Notwithstanding- their 

 number, they are of the least ecological importance. Their slender 

 stems occupy little space and take no part in sand-binding, while the 

 very existence of the entire group is due to the presence of the bunch- 

 grasses, wdiich act as windbreaks and hold the sand. H the number 

 of grasses decreases somewhat, there is a correspondingly larger 

 number of interstitials, but if the bare spots become too large, so that 

 blowing of the sand begins, their number begins to decrease. 



The most abundant species of interstitials are Oenothera rhomhi- 

 petala, Ambrosia psilostachya, Linaria canadensis, Cassia Chamac- 

 christa, Monarda punctata, and Croton glandulosus, var. scptcn- 

 irionalis. 



