20 U.S. NATIONAL MUSEUM BULLETIN 275 



positae, Zygophyllaceae, Resedaceae, Chenopodiaceae, Plantagin- 

 aceae, Tamaricaceae, and Euphorbiaceae. 



Owing to the few species (one or two) representing each genus of the 

 Cruciferae in the Sahara, this family is not included with the above 

 account of generic dominance, which is based on only those genera con- 

 taining five or more species. Because of the large number of Saharan 

 genera comprising the Cruciferae, it is still regarded as one of the 

 dominant families. 



Endemism 



In the Libyan Sahara, the presence of vast spaces almost unfit for 

 life constitute barriers to the dissemination of species, and endemism is 

 particularly well developed. Some plants are endemic to particular 

 geographic formations (mountains, sand seas, and hamadas). Other 

 species have a wider range of endemism and are endemic to the entire 

 Sahara; therefore, all degrees of endemism are possible. 



Specific endemism attained in the Sahara is in the neighborhood of 

 25 percent of the total component plant species (162 endemic species 

 of vascular plants out of a total of 650 species). Ozenda lists 35 species 

 of endemic vascular plants in the northern Sahara, many of which 

 occur in Libya. 



Adaptation to the Desert Environment 



The problem of adaptation of plants to a desert climate involves 

 primarily an adjustment to long dry periods. A large proportion of 

 desert plants shorten their fife cycle and thereby suppress their aerial 

 parts during periods of drought. Others maintain their aerial parts 

 but have an arrangement of anatomical devices which tend to diminish 

 water loss by evaporation. 



In the Sahara, two types of vegetation can be distinguished accord- 

 ing to the length of their life cycles: temporary vegetation and 

 permanent vegetation. 



The temporary vegetation is divisible into annual plants and the 

 geophytes. Following the rains, annual plants appear suddenly and 

 develop with amazing rapidity, completing their life cycle, including 

 flowering and fruiting, before the soil becomes dry. The length of the 

 vegetative cycle is variable among the different species but generally 

 is about four months. Some species of the genera Boerhavia L. and 

 Tribvlus L. complete their development from germination to the 

 production of seeds in 8 to 15 days. Additional examples of Saharan 

 ephemerophytes include Convolvulus fatmensis Kunze, Launaea glome- 

 rate (Cass.) Hook, Schismus barbatus (L.) Thell., and Plantago albicans 

 L. These ephemeral plants are important as sources of food for 

 domestic animals. 



