- 73 



assimilation being taken over by the two connate spiny pro- 

 phylJs. The whole plant forms a spiny ball sometimes as 

 large as 30 centimetres in diameter and of a grey colour. 

 (See fig. 66). 



Under the Summer-plants should also be grouped a red 

 Lichen, Lecidea decipiens which in some places is common 

 on the surface of the loess. 



The structure of the Summer-phanerogams will be dealt 

 with later (chap. 13), when the different types of desert-plants 

 are described. Here it is only necessary to give some of the 

 more important features. The species which grow in the most 

 favourable localities, where the ground-water is not too deep, 

 are generally these with relatively the richest foliage: Tama- 

 rix, Halimodendron, Prosopis, Peganum, Zygophyllum, Pluchea, 

 Innla, Alhagi have all distinctly green leaves and flat, with 

 the exception of those of the Tamarix. None of them however 

 have much foliage, nor does it cover the stems. In Alhagi 

 only the oldest leaves persist, so that the upper shoots look 

 like leafless spiny branches. 



The plants of the dry desert may be grouped as follows: 

 Succulents, Bracteole-Succulents or Leaf-Succulents, the last 

 including Salsola Arbuscula, subaphylla, rigida and verrucosa, 

 Reaumuria; leafless Stem-Succulents such as Haloxylon, Ana- 

 basis and Calligonum; deciduous shrubs like the species of 

 Astragalus where other organs take over the work of assimi- 

 lation; and finally plants with narrow leaf segments coated 

 with hairs (Artemisia). All the species from the dry clay- 

 desert belong to very xerophytic types, many have in addition 

 a halophilous stamp (cylindrical assimilation - organs with 

 aqueous tissue in the middle). In a case such as we are now 

 considering it may be difficult, perhaps impossible, to dis- 

 tinguish between the xerophytic and the halophytic; which 

 structural adaptations are due to desiccation and which to 

 salinity of the ground-water can only be positively determined 

 by experiments. 



The usual aspect of the clay-desert in summer is a flat 

 or slightly undulating surface, brown and dry, here and there 

 with slight incrustations of salt, bare or scantily covered with 

 scattered xerophilous plants which are herbs or small shrubs 



