406 GENERAL MORPHOLOGY OF PLANTS 



under peculiar conditions to stouter structures, which may be 

 quite large, or others reduced to scale-like bodies. The tissues 

 are very highly developed, and the presence of true vessels also 

 marks the Angiosperms off from most of the Gymnosperms. 



THE FLOWER.* 



554. The flower represents the highest stage in evolu- 

 tion of plants. The flower is the most characteristic feature of 

 the Angiosperms. It was foreshadowed in the higher fern plants 

 where spore-bearing leaves (the sporophylls) are massed together 

 at the end of a shoot, forming a strobilus. The Gymnosperms 

 show an advance upon the strobilus of the fern plants, because 

 of the development j^f the ovule, which encloses permanently the 

 macrospore and secretes a viscous substance which holds the 

 pollen grain (microspore) in position where it can germinate, so 

 that the sperms may be carried to the egg. In addition to the 

 ovule, and the contrivances for enabling the sperms to reach the 

 egg, the flower of the Angiosperms presents several important 

 features which indicate an advance in plant evolution. These 

 may be enumerated as follows : 



First. The appearance of a perianth. The development of 

 floral envelopes (sepals and petals) which serve to protect the 

 essential organs (stamens and pistils) in the bud. 



Second. The showy perianth. The showy character of the 

 floral envelopes, because of size and color, which serves to attract 

 insects to aid in pollination. In some cases where the flowers 

 lack a showy perianth there are colored bracts or leaves just 

 below the flower, or cluster of flowers, which serve the same pur- 

 pose, as in the flowering dogwood, the dwarf cornel or bunch- 

 berry, the painted cup, etc. A striking example is the Poinsettia 

 grown in greenhouses or conservatories. This coloration of 

 bracts and leaves at the base of the flower suggests how the 

 perianth may have been developed from leaves. 



Third. Dimorphism, and special adaptation. Monoecism, 

 dicecism, dichogamy, and the many peculiarities in structure of 



* Paragraphs 554-568 are for reference or special assignment. 



