GROUPS OF PLANTS. 



83 



Beginning with tiie germination of the seed we may outhne 

 the Hfe history of the plant as was done under Gymnosperms. 

 The seeds in the two groups are much aUke with the exception 

 that in the Angiosperms they usually have two integuments. 

 Within the Angiosperms two classes of embryos are distin- 



FiG. 53. Development of pollen sacs (microsporangia) in several of the Angiosperms: 

 A, showing beginning of archesporium (a), an outer sterile layer (b), position of connective 

 (con); B, later stage showing development of fibrovascular tissue (gf); C, longitudinal 

 section of archesporium; D, E, F, successive later stages showing in addition pollen mother 

 cells (sm) and tapetum layer (t). G, H, diagrammatic sections of mature pollen sacs show- 

 ing pollen mother cells (pm), tapetum (t), endothecium (end), exothecium (ex), and in 

 H longitudinal dehiscence with formation of what appears to be a unilocular pollen sac 

 on either side of the connective. — ^A-F, after Warming; O-H, after Baillon and Luerssen. 



guished, which give rise to the most important division of this 

 group of plants. In the one case a single cotyledon is formed at 

 the apex of the stem, and all plants having an embryo of this kind 

 are known as monocotyledons, that is, plants having one seed 

 leaf. In the other case two cotyledons arise laterally on the stem 



