Australian Flowering Plants. 189 



In the succeeding sections the information supplied already 

 in the earlier pages is assumed. 



(a) The Tropical Problem. 



(1) Distribution of Acacia, Cassia, Xanthoxylum, Vernonia. 



In the fertile tropics there are many large families of flower- 

 ing plants containing a great wealth of genera and species, 

 which are more or less luxuriant in habit, primary or gen- 

 eralized in character, and having a cosmopolitan range in the 

 fertile tropics. In the neighboring extratropical areas many 

 types occur secondary to the primary forms of the tropics. 

 These secondary forms are mainly xerophytes, and occur botb 

 as modifications of the cosmopolitan genera of luxuriant type 

 and as local genera grouped round the local forms which the 

 main cosmopolitan genus may have assumed. These secondary 

 sections, subgenera, or groups of genera, considered as distinct 

 local groups, appear to have relations more intimate with the 

 cosmopolitan tropical type than with each other. The secon- 

 dary types in the southern hemisphere are mainly undershrubs, 

 shrubs, or dwarfed trees, more rarely herbs, the leaf surfaces 

 are reduced and the evidence is that the specialization has been 

 in the direction of providing against excessive transpiration of 

 moisture. The secondary types in the northern hemisphere 

 are peculiar in that they generally are herbs, either annual or 

 perennial, more rarely trees, while the primary forms in the 

 tropics most frequently are trees of beautiful appearance and 

 luxuriant habit, with pinnate leaves. 



Examples are to be found in families such as the Myrtacese, 

 Leguminosse, Eutacese, Rubiacese, Sapindacese, Euphorbiacese, 

 Solanacese, Verbenacese, Orchidacese, Labiatse, Malvacese and 

 Sterculiacege. 



Before discussing any of these families individually it 

 would be advisable perhaps to consider the distribution of a 

 few of the most important genera in these and allied groups, 

 genera, for example, such as Acacia (or better still Mimosa 

 in the Linnean sense) Cassia, Vernonia, and Xanthoxylum. 



Of these Acacia is a peculiar type. Its primary form is 

 suggested by the section Grummiferse, which is spread over the 

 tropics and subtropics of the world, but occurring not so much 

 in the fertile tropics, as on the plains and wastes of the warmer 

 world. Indeed it is one of those remarkable types which 

 suggest strongly that in the days when some great connection 

 existed between all the great land blocks of the world within 

 the region of warm climate, that xerophytes had already come 



