THE DICOTYLEDONES 1937 



West Indies, while C. Uberica, Liberian coflFee, is also cultivated though on 

 a less extensive scale. From 1850 to 1880 coffee formed the main agri- 



FiG. 1864. — Coffea arabica. Flowers and fruits. Ceylon. From a 



commercial photograph. 



cultural crop in Ceylon but was largely destroyed as a result of attack 

 by the rust-fungus Hemileia vastatrix. 



Closely allied to Coffea are a group of genera grouped together in the 

 tribe Psychotrieae which are biologically interesting. The genus Psychotria 

 itself contains about 600 species widely distributed in the tropics. Many 

 possess small nodules on the leaves (Fig. 1865) containing bacteria which 

 are thought to fix atmospheric nitrogen. The genera Hydnophytum, which 

 comprises thirty species in eastern Asia, New Guinea and Fiji, and 

 Myrmecodia (Fig. 1866) with twenty species in Indo-Malaya, are epiphytes. 

 In both of these genera the base forms a large tuber which is anchored 

 to the support by adventitious roots. This tuber is mainly composed of 

 cork and is penetrated by numerous passages inhabited by ants. These 

 passages are formed at an early stage in the hypocotyl which swells into a 

 small tuber, in which a hollow axial cylinder of phellogen is formed which 

 cuts off cork on its inner side and parenchyma on its outer side, thus 

 increasing the size of the tuber while at the same time forming a hollow 

 space lined by cork. On the outside of the tuber there is a second phellogen 

 which cuts off cork outwards in the usual way. As the tuber grows more 

 phellogens appear, cutting off more internal cork and thus producing more 

 chambers in communication with the older ones. Though these cavities 

 are inhabited by ants it is not known whether they serve any useful purpose 

 to the plant. The tuber itself may be merely a water-storage organ. 



