THE ANGIOSPERMAE : HIGHER FLOWERING PLANTS 781 



waU. A single ovary with its enclosed ovules, together with a short column 

 of ovary tissue, called the style, which often supports the stigma, form a 

 morphological unit named the carpeL 



In the embryo-sac there are no archegonia, and no prothallus is formed 

 before fertilization. Only one oosphere nucleus is present, and normally 

 only one embryo is formed. A nutritive tissue develops in the embryo-sac 

 during the growth of the embryo and is called endosperm. 



Structures of the most varied kinds, which facilitate the dispersal of seeds 

 and fruits, are produced by post-fertilization developments from the ovules 

 and carpels. 



The vegetative forms displayed by Angiosperms are most extraordinarily 

 varied and show a very striking contrast to the rather limited and monotonous 

 aspect of the Gymnosperms. They assume every facies known in other 

 groups, from the thalloid type upwards, as well as many peculiar to them- 

 selves. Along with this morphological diversity goes the greatest adaptability 

 to habitats, which range through every climate and every sort of situation. 

 During the comparatively short period of geological time which has elapsed 

 since the Angiosperms first appeared in the Cretaceous period, they have spread 

 and multiplied until they have eclipsed all other groups in their importance 

 and have become the dominant race. Yet it seems that all the peculiarities 

 of form, appearance and function which have made possible their conquest of 

 such manifold difficulties can be traced to the three categories of organs, stem, 

 root and leaf, which have shown in this group an unequalled degree of plasticity. 



Anatomically the Angiosperms are notable for the presence, except in a 

 very few cases, of xylem vessels, often of large size. Centripetal xylem has 

 disappeared, and with it has gone the truly cauline vascular system which 

 characterizes the Pteridophyta and of which traces survive in the lower 

 Gymnosperms. All the stem bundles are common to stem and leaf. They 

 may pass through more than one internode of the stem, but sooner or later 

 each bundle passes into a leaf. We have argued in a previous chapter that 

 this implies a radical difference of organization between the Seed Plants and 

 the Pteridophyta and that no direct comparisons of their stelar structures are 

 valid (see p. 646). 



Many Angiosperms are annuals, living for only a single season. They 

 are of rapid growth and have little or no secondary thickening. This new 

 departure in plant habit aids rapid multiplication and dispersal and is, above 

 all, useful to plants which are pioneers in open ground, but it does not usually 

 enable them to hold for long the ground colonized. Like an armv of 

 nomads, they are ever on the move. 



Another type of habit not found in Gymnosperms is that of the geophyte, 

 the plant which perennates by means of underground stems and only sends 

 up a temporary flowering shoot. 



The Angiospermae are divided into two series : Dicotyledons and 

 Monocotyledons. The names are founded on a difference in the number 

 of embryonic leaves or cotyledons in the seedling, but this apparently minor 

 distinction is associated with a number of others and undoubtedlv marks 



