ON THE EFFECTS OF EXCESSIVE DEOUGHT UPON PLANTS. 507 



The heredity of xerophytic characters, like those of hydrophytes, 

 may be partial or complete. Thus the fleshy character of Cactaceous 

 plants is permanent and is seen in seedlings raised in England, as 

 are also their spines; but many spinescent plants, if raised in wet con- 

 ditions, may reproduce them for a season or two, but the spines 

 subsequently revert to branches, as in furze, rest-harrow, sloe, pear, 

 &c. In a wet spring, the spines of barberry are represented by true 

 leaves. 



Xerophytic Associations. — These are of various kinds, but all 

 are characterized by a small annual rainfall. The thorn-forests and 

 savannahs of tropical regions are succeeded by deserts and veldts. 

 Maritime regions, and mountains, when approaching the zone of 

 conifers, and high alpine regions, ^s well as arctic and antarctic zones, 

 are all xerophytic. 



What is noticeable is that similar features are found in the plants 

 of all of them — some of one kind, some of another — such as a great 

 decrease in size when one, a xerophytic species, is compared with 

 another of the same genus growing in a more favourable place. 

 Besides a dwarfing, the arrest of the stem is common, giving a tufted 

 habit. In extreme conditions, as those of arctic regions, what are 

 annuals in lower latitudes become perennials in higher zones. Just 

 as hydrophytes are all characterized by simil,ar structures of various 

 kinds, so are xerophytes, proving that the visible, as well as the 

 microscopic structures, in all plants are the result, with heredity, of 

 the responsive power of protoplasm to the external conditions of life. 

 This is the only true and universal explanation of evolution.* 



* Those who wish to pursue this subject of ecology should study especially 

 Dr. Schimper's Plant Geografhy and Warming's (Ecology. My Origin of Plani 

 Structures and an Introduction to Plant Ecology are smaller. 



