xxxviii FLORA OF TASMANIA. [Tropical Flora. 



§ 5. 



On the Tropical Australian Flora. 



There are no geographical or other features of the Australian continent which enable me to 

 draw any natural boundary between temperate and tropical Australia. In selecting a botanical tropic 

 of Capricorn, I hence have had recourse to the distribution of the plants themselves, and these must 

 afford very vague data. The tropical Flora, in one form, advances further south on the west coast 

 and on the central meridian than on the east, because of the absence of mountains, and hence of 

 water, on the west, which causes combine to favour the prevalence of hot, desert types of vegetation, 

 many of which advance even to Swan River. On the east coast again the climate is moister, and we 

 hence not only find the most marked features of extratropical Australian vegetation, — Stackkousia, 

 Boronia, Tetratheca, Comesperma, various genera of Epacridece, Leguminosce, Myrtacece, etc., ad- 

 vancing in full force as far north as Moreton Bay, lat. 27°, which I have somewhat arbitrarily 

 assumed there to be the limit of the temperate Flora, — but Palms and other tropical forms run- 

 ning down the coast almost to Bass's Straits. To the northward of Moreton Bay (judging especially 

 from Mr. Bidwill's Wide Bay collections) not only do many temperate forms disappear, but tropical 

 ones, — Malvaceae, Sterculiacece, Acanthacece, Euphorbiacece, Convolvulacece, Meliacece, and Sapin- 

 dacece, Ficus, together with numerous tropical Indian weeds, — become a prevailing feature in the 

 landscape. The Araucarias, according to M'Gillivray (Voy. Rattlesnake, 1846-50), begin at Port 

 Bowen and advance to Cape Melville. Pandanus, according to the same authority, commences at 

 Moreton Island. 



On the west coast I am puzzled where to draw the line. Judging from Drummond's her- 

 barium, formed between the Moore and Murchison rivers (lat. 27° 30' S.), the vegetation is there still 

 typically that of the Swan River, though much modified, and reduced greatly in number of genera 

 and species. Sir G. Grey, in his adventurous journey from Port Regent to Swan RiverJ enumerates 

 various eminently tropical forms as occurring to the north of Sharks Bay (lat. 26° S.), as Nutmeg,* 

 Araucaria* Calamus (abundant), Vines, many Figs, and Areca, together with a Banksia of Swan River, 

 which he distinctly alludes to as being quite exceptional (p. 247). To the southward of Sharks Bay 

 again, he met with Xanthorrhoea and Sow-thistle,t both of whose northern limits he gives as 28° S., 

 and Zamia (lat. 29° S.). The parallel of Sharks Bay, I have hence assumed to be north of the posi- 

 tion of the tropic of vegetation. 



In determining what may be called the tropic of vegetation, regard must be had not only to the 

 latitude and isothermal lines, but to the abundance of the vegetation and its character : and, indeed, 

 in such a country as Australia the latter elements are perhaps of the greatest importance, owing to 

 the diminution northward of so many peculiar genera that make up a large proportion of the 

 extra-tropical vegetation, and to the fact that the tropical Flora is so very poor in number of species, 

 and deficient in such conspicuously tropical genera as Epiphytic Orchids, Palms, Ferns, Scitaminem, 

 etc. etc. 



Taking all elements into consideration, of the vegetation, actual temperature, and relative hu- 



* 1 find no notice elsewhere of these genera being found on the west coast, and suspect some error, 

 t Leichardt mentions the Sow-thistle as abundant in lat. 25° 30' S. on the Gilbert range. 



