SEASONAL DIMORPHISM OF DESERT AND 



MEDITERRANEAN CHAMAEPHYTES AND ITS 



SIGNIFICANCE AS A FACTOR IN THEIR WATER 



ECONOMY 



G. Orshan 

 The Hebrew University, Jerusalem 



INTRODUCTION 



Although chamaephytes as a life form characterise high latitudes and high 

 altitudes where temperature is the limiting factor, they also play an 

 important role in steppes and deserts (Raunkiaer, 1934; Cain, 1950). In the 

 Middle East they are the leading species of both desert and certain Mediter- 

 ranean plant communities. 



Seasonal dimorphism is a most common feature among the chamae- 

 phytes and is generally associated with a seasonal reduction in their 

 transpiring body. Most of them show a certain amount of growth and 

 development throughout the year, in spite of the dry and hot summer, but 

 this growth yields different kinds of leaves and branches in different seasons 

 which are in turn periodically shed. At present we know very Httle about 

 the mechanisms through which environment affects the growth and form 

 of chamaephytes on the one hand, and the ways through which their 

 particular form and growth types serve as an adaptation to the environment 

 on the other. It seems that even the problems have not yet been properly 

 defmed. 



The present paper deals with aspects of the seasonal dimorphism of 

 chamaephytes. Their phenology and growth habit will be described, and its 

 effect on the extent of the seasonal reduction in their body accounted for. 

 In addition, data will be presented showing the annual march of their 

 transpiration rate. An attempt will then be made to correlate these aspects 

 and to fmd out what their relative importance is in the water balance of the 

 chamaephytes. 



During the last few decades, the transpiration rates of a considerable 

 number of chamaephytes has been measured in Isreal, in their natural 

 habitat, by the cut leaf quick-weighing method (Evenari andRichter, 1937 ; 

 Orshansky, 1952, 1954; Shmueh, 1948; Zohary and Fahn, 1952; Zohary 

 and Orshan, 1954, 1956; Zohary, 1955). Recently the annual march of the 

 weight of the transpiring body of some chamaephytes was determined 



