swingle: new genus of citrous fruits 569 



The porous lime unites very readily with dry carbon dioxide, 

 and the compound dissociates readily with rising temperature. 

 The crystalline lime unites very slowly with dry carbon dioxide. 

 The crystalline forms of calcium carbonate dissociate very slowly 

 at low temperatures, and the rate does not seem to be hastened 

 by the presence of Fe203 or of CaO. Aragonite is transformed 

 into calcite within an hour at 425° in the vacumn furnace. The 

 dissociation pressures of crystalline calcium carbonate at 400° 

 are of the order of magnitude of 0.003 to 0.009 mm. 



BOTANY. — Microcitrus, a new genus of Australian citrous fruits. 

 Walter T. Swingle, Bureau of Plant Industry. 



In the first quarter of the nineteenth century Allan Cunning- 

 ham collected in Australia and sent to Europe scanty specimens 

 of a plant which at first was referred by botanists to Limonia 

 and later to Citrus. Four species of Citrus in all have been 

 described from Australia: C. australis Planchon, C. australasica 

 F. Muell., C. inodora Bail., and C. Garrowayi Bail. 



In the course of a systematic study of the species of Citrus 

 and related plants the writer has been able to examine these 

 Australian citrous fruits both as dried specimens in the principal 

 European and American herbaria and as live plants in Italy 

 and in the greenhouses of the Department of Agriculture at Wash- 

 ington, D. C. It soon became apparent that they differed from 

 the other species of Citrus in a number of characters of impor- 

 tance in this group. The dimorphic foliage showing marked con- 

 trast between the juvenile and mature forms, the minute flowers 

 with free stamens and very short pistils, the parallel venation 

 of the leaves and their very short wingless petioles, and the 

 few-celled fruits, with subglobose stalked pulp-vesicles and small 

 rounded seeds, give these plants a very different aspect from 

 the commonly cultivated species of Citrus. Furthermore, in 

 greenhouse cultures the young seedlings show cataphylls like 

 Eremocitrus and Poncirus, instead of a pair of subsessile broadly 

 oval or ovate leaves as in Citrus. In view of these important 



