258 



the large number of spring-plants with light-shoots which 

 only live a short time, and also the chamaephytes and 

 fanerophytes on which annual shoots are common, but 

 where the persistent shoots of most species lose their distal 

 parts. 



2. Reduction of the leaves. It is a well-known feature 

 of deserts that the leaves become small or disappear entirely. 

 As a rule the stem assumes the part of vicar, it may be in 

 some cases where leaves are still present, as assistant vicar, 

 or in other cases, where leaves are entirely wanting, as 

 deputy-vicar (B. JONSSON 1910). BOIRIVANT has proved by 

 experiments a correlation between absence of leaves and 

 formation of assimilating tissue in the stem. 



3. The frequency of the centric type of assimilating 

 organs (in and outwith the Chenopodiaceae) and of isolateral 

 leaves. Isolateral structure seems to be specially dependent 

 on strong light (HEINRICHER), and it must promote the process 

 of assimilation. The xerophytic structure of the assimilating 

 organs is likewise a well-known feature in desert plants as 

 recorded in the various text-books on plant-geography; see 

 also HENSLOW 1893, with whose interpretation, however, I 

 do not agree. 



SECTION IV. THE FLORA OF THE 

 TRANSCASPIAN LOWLANDS. 



CHAPTER 14 



The Elements of the Flora. 



When at the beginning of the Quaternary period Trans- 

 caspia emerged from the sea which retreated towards the 

 West and North, a change in climatic conditions took place 

 simultaneously in the various parts of Western Asia previously 



