sect, n PHANEROGAMIA 445 



with the exception of the Ferns, Horse-tails, and Club Mosses, all our 

 foliage trees belong to this class. Varying in size from plants like 

 Wolffia arrhiza, no larger than the head of a pin, to the Eucalyptus 

 trees of Australia with a height of 140-150 m., they exhibit a great 

 diversity of external form, greater than in any other class of the 

 vegetable kingdom. 



The greatest variety of form appears, however, in the structure of 

 the flowers. It is the flowers that distinguish the Angiosperms so 

 markedly from the Gymnosperms, and, together with the fruit and 

 seeds to which they give rise, they furnish the most available means of 

 classification. 



Little of general application can be said concerning the vegetative 

 organs ; they will be considered more in detail in treating of the 

 separate sub-classes, orders, and families. Decided differences between 

 the external differentiation of the Angiosperms and the Gymnosperms 

 are not apparent. As regards their internal structure, the Angiosperms, 

 in contrast to the Pteridophytes and almost all Gymnosperms, possess 

 true vessels, except in the case of certain Magnoliaceae, which in their 

 secondary growth resemble the Conifers (cf. p. 128). 



The Flower 



While the Gymnosperms have only simple, inconspicuous flowers, 

 in which a perianth is either entirely absent or only represented by 

 scale-like cataphylls, the flowers of the Angiosperms have a more com- 

 plicated and varying structure, and in most cases are provided with a 

 well-developed, coloured perianth. This difference in the character 

 of the flowers of the two classes is due, in great measure, to the modi- 

 fications which have arisen in the flowers of the Angiosperms during 

 their transition from wind- to insec1>pollination (cf. p. 281). The 

 involuntary intervention of insects in transferring the pollen from 

 flower to flower disturbed the formative force of the flowering region, 

 and called forth that wonderful degree of adaptation displayed by the 

 flowers of so many Angiosperms, rendering them the most remarkable 

 structures in the vegetable kingdom. 



Although the influence of the insect-world upon the formation of 

 the flowers is perceptible in the great majority of Angiosperms, the 

 pollination of a few of the lower groups is still effected by the wind ; 

 others again have returned to that condition, or have, although rarety, 

 resorted to self-pollination. In such cases the flowers are inconspicu- 

 ous and odourless, for both colour and perfume are only of use to plants 

 as a means of enticing insects. While the possession of conspicuous 

 or sweet-smelling flowers is a sure indication of the Angiospermic 

 nature of a flower, it must not be concluded, conversely, that plants 

 with insignificant flowers do not belong to the Angiosperms. 



Morphology of the Flower. — In contrast to the Gymnosperms, 



2 i 



