A NGIOSPERMS 4 1 7 



higher fern plants (related to Selaginella, etc.). The two classes 

 of seed plants, the Gymnosperms and Angiosperms, probably 

 represent divergent lines of development from the same or related 

 stock belonging to the Pteridophytes of geologic times, that is of 

 a very early past, for there are fossil remains of both at a very 

 early period. Some of the characters possessed by the Angio- 

 sperms which mark an advance over those of the fern plants are 

 possessed by the Gymnosperms also. Taking the Angiosperms 

 alone (see paragraph 551 for advances in the Gymnosperms) the 

 principal advances may be enumerated as follows: 



First. The more highly developed tissue systems in the parts of 

 the plant body, root, stem and leaf, with a more efficient con- 

 ducting system with true vessels. 



Second. The development of the parts from a growing point, 

 or meristem, consisting of a group of cells, instead of from a 

 single apical cell. 



Third. The collateral arrangement of the tissues in the vascu- 

 lar bundles; in the dicotyledons the cyclic arrangement of open 

 bundles which permits of indefinite growth of the stem resulting 

 in massive trunks (also a character of many of the Gymnosperms) . 

 In the monocotyledons the bundles are scattered in the funda- 

 mental tissue of the stem, which soon pass over into permanent 

 tissue and the development of massive tree trunks is limited. 



Fourth. The great variety of vegetative forms, trees, shrubs 

 and herbs, with their adaptation to an aquatic as well as a land 

 habit, and, especially in the latter, the adaptation to a great variety 

 of climate and soil fits them for growing in climates from the 

 tropic to the subarctic regions, in swamps, deserts, plains and 

 steppes, as well as in regions more temperate as regards climate 

 and moisture content of the soil. Many are adapted also to 

 grow as shade plants or climbers. With this great variety of 

 habit and character the Angiosperms are especially fitted to 

 compete successfully with climate, soil, and with other groups of 

 plants. 



Fifth. The absolute prevalence of heterospory which insures 

 cross fertilization as far as the gametophytes are concerned, but 

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