PECULIARITIES OF THE CAPE FLORA. 



113 



water ; hence plants must make the best use of it when the rainy season 

 comes. 



Secondly, they must so construct the superficial tissues as to secure as 

 little loss as possible by transpiration during the rainless season. 



Thirdly, they must, if possible, be able to absorb dew and mist during 

 the months when no rain falls. 



Lastly, they must be able to store up water within their tissues when- 

 ever it can be obtained. 



Roots. — The roots are, of course, the main absorbing organs, and 

 the chief point to notice is that when plants grow in hot sandy districts 

 in which the water can remain at some less or greater distance below T the 

 surface, the roots will elongate proportionally, even to the extent of forty 

 feet, as in the case of the Naras plant (Acanthosicyos horrida, of the 

 order Cucurbitacece), which grows near Walfish Bay, on the west coast of 

 Damaraland. Similarly small herbs, as Monsonia nivea in North Africa, 

 which produces a little tuft of leaves, may have a root some twenty inches 

 in length. Dr. Aitchison * describes " several species of Astragalus which 

 have long whip-like roots the bark of which is employed as twine by 

 the people of Beluchistan." 



This elongation of the tap-root is due to the sensitiveness of the 

 growing point to moisture, which induces it to develop in the direction 

 from which watery vapour ascends. Such is often the case in England 

 when the appropriate conditions occur ; such induce long roots to be formed 

 in plants growing over a cave which find their way down and hang 

 suspended in the moist air. They may enter a field drain-pipe, as a 

 turnip root in the writer's possession, which grew to upwards of six feet 

 in length. 



Roots may also act as a storage of water. In some desert plants in 

 North Africa such has been found to be the case. Thus in the genus 

 Erodium there are three desert species which develop tuberous structures 

 in the roots, which proved to be water- and not starch-bearing structures. 

 In some Grasses the cortical tissue of the roots stores it up. The probable 

 cause of the enlargement of the cells in roots is the great heat of the 

 sand, which sometimes amounts to 130° F. ; for M. E. Prillieux has shown 

 experimentally how an abnormal excess of heat affected the roots of 

 Beans Sec. in a precisely analogous manner by enlarging the cells of the 

 cortex and pith. While, however, in that case the distension of the 

 parenchymatous tissue was, of course, abnormal and pathological, in desert 

 plants it has become a characteristic and an important feature. 



Stems. — A common feature of dry desert land is for the woody stems 

 to be gnarled, stunted, twisted, &c, with great disarrangement and 

 malformation of the various internal tissues. This is due to the arrest 

 of development and consequent degradations which are found in the 

 structure of the leaves, upon which that of the stem and roots depends ; for 

 the leaves possess a much reduced assimilative power for making wood. 

 Consequently there are often various anomalies in the distribution of the 

 histological elements in stems of desert regions. 



Another type, well illustrated in the South African flora, is of a thick, 



* A Summary of the Botanical Features of the Country traversed by the Afghan 

 Delimitation Commission, 1884. 



K 



