— 166 - 



desert; as the distribution of the growth-forms of the species 

 of larger families are here given as percentages. The orders 

 which are especially represented by annual species are the 

 Cruciferae and the Chenopodiaceae, the percentage of these 

 being respectively 90 and 72. Of these the Cruciferae are 

 all early flowering ephemerals, whereas about 82 per ct. of 

 the annual Chenopodiaceae are summer-plants which flower 

 after the beginning of July. A large proportion of the early 

 flowering annual Chenopodiaceae also live far into the summer. 



These two families, the Cruciferae and the Chenopodiaceae 

 are thus typical representatives of two widely different 

 series of adaptations to desert-life: the one shows itself in 

 the quick development of plants of mesophytic structure 

 during the favourable season; the other is characterised by 

 slow development combined with the power to withstand the 

 unfavourable conditions of the dry season. The former series 

 include all the other families in table 5 with a large number 

 of annual plants, e. g. Borraginaceae, Gramineae and Ranuncul- 

 aceae. Outside the Chenopodiaceae there are very few annual 

 summer-plants. 



That the adaptation of the ephemeral plants is advan- 

 tageous is easily seen; the continuity of the race is ensured 

 with a slight expenditure of material. Life with these plants 

 seems a much simpler matter than in the case of the annual 

 summer-plants which have to contend with a long hot 

 summer before their seeds are ripened. The existence of the 

 latter is far more expensive, because for one thing material 

 must be produced for the development of mechanical, water- 

 storing, and other specialised tissues which the ephemerals 

 do not require; moreover many of the annual summer-plants 

 do not complete their natural span of life because they are 

 buried by the sand, or the soil blows away, or they die 

 of thirst. 



In this connection attention is drawn to the outstanding 

 difference between the mode of occurrence of the ephemerals 

 and the annual summer-plants. As already described, the 

 former often appear in masses during spring, while the latter 

 almost always occur as single plants standing widely apart. 

 This is of course largely due to the greater number of 



