(g) Brachyblastic leaf shedders. This type comprises the majority of chamae - 



phytes, the dominant life form in desert vegetation, though constituting a small 

 percentage only of the desert flora. The new shoots produce in the axils of 

 the winter leaves small bud- like branches (brachyblasts) which, after the shed- 

 ding of the winter leaves, are active during the whole summer {Reaumuria 

 type). 



(h) Aesticladous leaf shedders. In this type the brachyblasts develop in summer 

 and consist of a spiny axis and minute leaflets, all dying away at the end of 

 the dry season (e.g. Noaea mucronata). 



(i) Articulate shoot splitters. These are articulate evergreen stem succulents in 

 which considerable parts of the green 'skin' of the last year's stems are dry- 

 ing off and then split into rings and fall down. Herebelong Anabasis articu- 

 lata, Haloxylon articulatum, etc. 



(j) Articulate branch splitters and shedders. As above but a part of the last 



year's branches also break down and fall away in summer {Haloxylon persicum 

 type). 



This way of morphological analysis renders more meaning to the life- form con- 

 cept. It shows that surface reduction of the transpiring body, achieved in various 

 ways, is most important for the permanence of the desert vegetation. 



Transpiration 



In order to obtain critical data on the transpiration behaviour of desert plants, 

 measurements of transpiration intensity of the leading species of the most typical 

 plant associations have been carried out throughout the whole year. 



Transpiration has been measured by the rapid- weighing torsion balance (Huber, 

 1927), whereoy excised plant parts have been exposed for 2-4 minutes. In most 

 cases two or more parallel measurements with the shortest possible interval bet- 

 ween them were made every hour for each plant. Reference has been made to fresh 

 weight (see Huber, 1*^27; Walter, 1051; Hygen, 1951) and figures have been calcu- 

 lated to hourly averages (mg/g.h. ). In order to simplify the presentation of results, 

 data of a single summer day (in most cases August) and a typical spring day have 

 been chosen for each plant (Fig. 3). 



The conclusions from own data and those obtained by Evenari & Richter (1937) 

 and Shmueli (1948) are: 



(1) Desert plants vary considerably in their transpiration behaviour, both quan- 

 titatively and qualitatively. 



(2) As to transpiration intensity two groups of plants can easily be distingui- 

 shed: those with a winter and those with a summer maximum. The former includes 

 the bulk of the permanent vegetation, while the second consists only of a few plants 

 with extraordinarily deep roots reaching sources of permanent moisture. The fact 

 that the dominant representatives of the permanent desert vegetation shows a con- 

 siderable summer decrease in transpiration intensity, is highly significant in the 

 water ecology of the desert. 



60 



