1776 A TEXTBOOK OF THEORETICAL BOTANY 



and passes out through the micropyle, and the united cotyledonary tube 

 which surrounds the plumule. The cotyledon is provided with a ring of 

 vascular bundles which are continuous with those of the hypocotyl and the 

 whole of its external surface is covered by absorbent cells which take up 

 food material from the endosperm. This food is entirely used in the develop- 

 ment of the hypocotyl which grows fatter and longer until it may be i or 2 ft. 

 long. Finally the vascular bundles between the cotyledon and the hypocotyl 

 are ruptured, the embryo falls away and its lower end plunges into the mud 

 and forms roots. Meanwhile the now liberated plumule grows out and 

 forms a stem with leaves. Later the prop-roots arise both from the 

 epicotyledonary stem and also from the hypocotyl. Later still the primary 

 root dies and its function is assumed by the prop-roots. It is interesting to 

 note that these are liberally supplied with lenticels which probably function 

 as pneumatothodes. 



The family is essentially a tropical one, occurring chiefly in Asia, 

 Australia and the East Indies. Rhizophora mangle occurs in America and 

 West Africa. 



Poga oleosa which occurs in French Equatorial Africa is the source of 

 the Poga Nuts which occasionally find their way into Britain. 



Hutchinson considers that the family shows affinities to the Malvales 

 and may provide a connecting link between the two orders. 



The Combretaceae are another family of tropical and subtropical 

 trees and shrubs which are of some economic importance on account of 

 their fruits. Terminalia chebiila and certain closely allied species provide 

 the Myrobalan Nuts which are an important article of commerce in India. 

 They are used for dyeing, tanning and also in medicine. T. glabra provides 

 good timber. Other genera form beautiful climbers some of which are in 

 cultivation. Combretum butyrosum yields a butter-like substance known in 

 Central Africa as Chiquito which is used as butter by the natives. 



We now come to the Myrtaceae which are a large and very important 

 family. It is not represented in the British Flora although many species 

 are commonly cultivated in gardens and greenhouses. It is essentially a 

 tropical and subtropical family, containing about ninety genera and over 

 2,800 species. They are all woody plants, many forming large shrubs or 

 trees, with entire, leathery leaves. The presence of bicollateral vascular 

 bundles is a constant feature in the group, and many species possess 

 spherical glands containing essential oils. These glands, which are formed 

 lysigenously, are developed not only in the leaves but also in the young stems 

 and in the floral organs and fruits. 



The flowers (Fig. 1667) are sometimes solitary in the leaf axils but more 

 generally they are produced in cymose inflorescences. They are herma- 

 phrodite, regular, actinomorphic and usually epigynous. The calyx 

 normally consists of either four or five sepals, alternating with four or five 

 free petals. There are a large number of stamens which are inserted in 

 whorls on the raised edge of the receptacle. The ovary may have one to 

 many loculi containing two or many pendulous, anatropous or campylo- 



