THE DICOTYLEDONES 1731 



stigmatic surfaces open out, occupying the same position as the anthers 

 did previously. When the styles separate they remain spirally twisted so 

 that the proboscis of an insect must touch a stigmatic surface from whatever 

 side it may approach the flower. If the insect has previously visited a 

 younger flower cross-pollination is thus ensured. 



The genera Silene and Dianthus are widely distributed, the greatest 

 number of species occurring in the Mediterranean region. The latter 

 genus prefers dry exposed situations. Species of Silene occur on the 

 mountains of Europe, tropical Africa and in Mexico, while those of Dianthus 

 are absent from the New World. Certain species of Silene and Lychnis 

 also occur in arctic regions. 



The family is of little economic importance. Saponin is obtained from 

 the roots and leaves of Saponaria officinalis, the Soap Wort, which is 

 sometimes used for washing. Among the garden flowers already mentioned 

 the genus Dianthus is the most important. Pinks are mostly derived from 

 D. plumarius, Carnations from D. caryophyllus and Sweet William from 

 D. barbatus. 



The anthers of various genera are subject to attack by fungi: Puccinia 

 anther arum, a Rust, and Ustilago violacea, a Smut. In both cases the anthers 

 fail to produce pollen grains but liberate fungal spores instead. Reinfection 

 is achieved by the insects normally concerned in cross-pollination. 



PROTEALES 



The Proteales are i\rchichlamydeae in which the flowers are either 

 bisexual or unisexual as a result of abortion of part of the flower. The 

 perianth is composed of one series of parts which is considered to be the 

 calyx. It is often brightly coloured. There are four stamens opposite the 

 calyx segments, the ovary is unilocular and the seed is devoid of endosperm. 



The plants are usually woody and many are trees, though some would 

 be better described as tall shrubs. The leaves are usually alternate, entire 

 or pinnate. There is a single family, the Proteaceae, which is distributed 

 in the drier parts of Australia and South Africa. It contains 960 species 

 distributed among fifty genera. 



We shall not consider the family in detail but will only refer to some of 

 the more interesting members. One of the best-known members, already 

 referred to in this book, is the genus Hakea which contains about 100 

 species, native to Australia. The seedlings show an interesting variation 

 in their foHage. The cotyledons are round and somewhat fleshy, while 

 the later leaves are either much divided or else form spiny needles. The 

 anatomical structure of these needles is centric, with a peripheral palisade 

 layer and large radial prop cells which span the assimilating tissues. Similar 

 centric leaves are found in species of Grevillea, which is also a large Austra- 

 lian genus with about 170 species. Several species are cultivated in 

 greenhouses in this country, the commonest being G. robusta, which grows 

 into a fair-sized shrub with fern-like foliage. In Australia this species grows 



