ARID PORTIONS OF SOUTH AUSTRALIA. 31 



CERTAIN CHARACTERISTICS OF THE VEGETATION OF 

 DRY REGIONS. 



In external and internal morphology, as well as in many physio- 

 logical processes, the vegetation of regions having a small rainfall is 

 different from that of the humid regions. For instance, the shoot may 

 be greatly reduced both as to size and surface. The constituent 

 members of the shoot may assume fairly vertical positions. The 

 foliage may be largely restricted to the ends of the branches, from which 

 may arise a canopy-form of shoot. The leaves may be wanting or they 

 may be replaced by phyllodia. Succulency may be found in leaf, shoot, 

 and root, or in any of these. Other modifications include the rolling 

 of the leaves of grasses, greatly elongated type of leaves, or phyllodia, 

 and in some forms dissected leaves in which the leaflets or lobes may be 

 considered the physiological equivalent of leaves. In many species 

 the shoots are provided with trichomes of various kinds, which serve 

 as a protection against rapid loss of water from the surface. The 

 trichomes may protect indirectly through the secretion of resinous 

 substances, which coat the surface, especially of young leaves or shoots 

 (Collins, 1918:255). 



The roots of xerophytes are as a rule deeply penetrating, but this 

 is not without exceptions. Many forms with water-storage capacity, 

 for example, have roots which lie close to the surface of the ground 

 (Cannon, 1911). Also, perennials may have roots of a dual habit in 

 that some are superficially placed, and some may penetrate deeply. 



The leaves (or their equivalent) of xerophytes are leathery in texture. 

 An examination of their structure shows certain characteristic features, 

 among which may be mentioned the following: The outer wall of the 

 epidermal cells may be heavy and heavily cutinized and sclerenchyma 

 is well developed. There is usually found palisade tissue and few 

 intercellular spaces. The stomata are protected in various ways, as by 

 being placed at the bottom of tubes, in which case the walls of the tubes 

 may be cutinized. Storage cells for water, which in times of need 

 yield water slowly to the adjacent cells, are often found. In large, 

 fleshy species, where the water-storage tissue is well developed, the 

 stored water may be sufficient to enable the plant to live for a period 

 exceeding 73 months in a dry atmosphere and without absorbing 

 additional water (MacDougal, Long, and Brown, 1915:290). 



As would be expected from the specialization of the structures of 

 xerophytes, as well as from the leading features of their morphology, the 

 physiological characteristics of the plants of the desert-arid regions 

 have many points of interest. These are largely associated with the 

 water relations of the plants; thus the effects of dryness have been 

 followed in many directions. The growth-rate is less at midday, when 

 the rate of transpiration is high and, relatively speaking, the rate of 

 water absorption by the plant is low (MacDougal, 1918:59). The 

 progressive desiccation of the soil and of the tissues in Opuntia versicolor 



