THE FLOWER 355 



any way. The further evolution of the flower has brought about 

 increased efficiency and precision in pollination, and appears to 

 have involved a gradual shortening of the floral axis, with the 

 result that the different sets of organs became whorled and 

 reduced in number. At the same time fusion of parts took 

 place to a more or less marked extent. Thus, the commonest 

 type of Dicotyledonous or Monocotyledonous flower possesses 

 two whorls of stamens (3 to 5 in each whorl), and a syncarpous 

 ovary formed by the fusion of a small number of carpels. 

 Another development appears to have been concerned with the 

 better protection of the ovary, the receptacle gradually becoming 

 more and more concave (progressive stages can be found in the 

 perigynous Rosaceae), until the ovary is completely enveloped 

 (epigynous flowers). Certain it is that the most highly specialised 

 families (Umbelliferae, Compositae, Orchidaceae) all possess such 

 epigynous flowers, with the different members in whorls. The 

 irregular (zygomorphic) flowers found in many families are no 

 doubt a special development connected with the perfection of 

 mechanisms for insect pollination (cf. p. 373). 



The classification of Angiosperms into Monocotyledons and 

 Dicotyledons is based on many morphological and anatomical 

 features (leaf-form, number of cotyledons in the embryo, stem- 

 and root-structure, and number of floral parts), the Monocoty- 

 ledons being an essentially herbaceous group exhibiting a high 

 perfection of means for perennation and hibernation (e.g. bulbs, 

 corms, etc.). The evidence that the Dicotyledons and Mono- 

 cotyledons were derived from a common stock is almost over- 

 whelming. This fact is plainly indicated, for example, by the 

 widespread traces of the presence of a cambium in Monocoty- 

 ledons, and the extremely few characters, either of development 

 or structure, that are entirely restricted to the one or the other 

 group (cf. also p. 367). 



The Angiosperms are the predominant vascular plants on 

 the earth's surface at the present day, and comprise some 125 ooo 

 distinct species. In Britain this preponderance is especially 

 marked. The native Angiosperms here number about 2,000 

 species and the Gymnosperms four species ; amongst the 

 Cryptogams, there are about 60 Pteridophyta, 600 Mosses, 250 

 Liverworts, several thousand species of Algae, and some 5,000 



