THE DICOTYLEDONES 



1703 



The flesh is white and the flavour is somewhat hke that of a plum but 

 more delicious. It only thrives in the region of the equator and is culti- 

 vated chiefly in the East Indies and Malaya. The fruit does not keep 

 well and is seldom exported. 



Garcinia hanbiiryi is the Gamboge Tree, a native of Siam, while Penta- 

 desma biityracea, the Tallow Tree, yields a valuable fat. Calophylhim 

 inophylhim yields Domba oil, and is known as the Alexandria Laurel. It is 

 widespread in the eastern tropics. Mammea americana, the Mammee 

 Apple, is a native of the West Indies. The fruits are eaten by the indi- 

 genous population but are of poor, subacid quality. 



In the family Dipterocarpaceae we may mention two plants of 

 economic importance: S/iorea robusta, the Sal Tree of India, a magnificent 

 timber tree which also yields Dammar resin; and Dryobalanops aromatica 

 which is the source of Borneo Camphor. This substance forms yellow 

 crystals between the wood elements. Several other genera also produce 

 oils of local economic importance. They occur mostly in India and the 

 East Indies. 



Finally we may briefly mention the Cactales or as they are sometimes 

 called the Opuntiales. The order includes the single family Cactaceae. 

 The systematic relationship of the 

 group is doubtful. Bentham and 

 Hooker consider it related to the 

 Passiflorales, while Engler places 

 it near the Myrtiflorae. Rendle in 

 his recent work follows Engler, 

 while Hutchinson considers that the 

 view of Bentham and Hooker is 

 more correct and relates it to the 

 Passifloraceae through the Cucur- 

 bitales. 



As a whole the Cacti show very 

 varied but remarkable adaptations 

 to a xerophytic mode of life (Fig. 

 1569). They are uniformly succu- 

 lent and range in size from small 

 plants like rosettes [Mammilaria) to 

 large columnar trees such as Car- 

 negiea gigontea which may reach a 

 height of 70 ft. by 2 ft. thick. A 

 few live as epiphytes on forest trees, 

 as for example Rhipsalis, while 

 others again form an impenetrable scrub, e.g., Oputitia, with flattened stem 

 segments. Species of this genus became such a plague when introduced 

 into Australia that considerable areas of Queensland became uninhabitable 

 and the extermination of this Cactus, the Prickly Pear, provides one of 

 the most remarkable stories of Biological Control (see Volume IV). 



Fig 



1569. — Melocactus communis, with 

 young fruits on top. 



