VEGETA TIVE REPROD UCTION. 



245 



placenta in angiosperms commonly consists of a cushion of 

 tissue usually at the united edges of the carpel or carpels. If 

 the carpels are united into a compound pistil, the placentas 

 will be either isolated, as ridges upon the inner face of the 

 wall of the ovulary (fig. 252), 

 or aggregated at its center 

 (fig. 251). Occasionally the 

 ovules arise upon the entire 

 inner face of the carpels, as in 

 the gentians. 



Fig. 257. 



Fig. 258. 



Fig. 257.— A median longitudinal section through the flower of Rheum undulatum. 

 s, a sepal; />, a petal; a, a, n, anthers; n, stigma; /, ovulary; kk, sporangium, 

 which, with the two integuments over it, forms the single ovule terminating the axis ; 

 dr, nectary. Magnified about 10 diam. — After Sachs. 



Fig. 258. — Pimpernel \Amtgallis arvensis). A, median longitudinal section of a 

 young flower-bud ; /, sepal ; c, corolla, just beginning to develop ; a, anther ; K, car- 

 pels growing over S, the apex of the axis. B, median longitudinal section of the 

 pistil, c, the carpels, forming a roof over S, the axis on which ovules are beginning 

 to develop, and growing up to form a columnar style at whose apex is the stigma, «. 

 C, the same, older. S, the enlarged apex of the axis showing six ovules, SfC, in sec- 

 tion ; gr, the style ; n, the stigma, on which are lodged pollen grains, /. All magni- 

 fied. — After Sachs. 



344. Stamens. — A stamen is a leaf (sporophyll) of the 

 seed plants which bears the microsporangia, or pollen sacs. 

 The flowers whose essential organs are all stamens are said to 



