208 Plant Biology 



17. The flowers, which are distinguishing characters of the entire group 

 of angiosperms, show great diversity of structure. Flowers are concerned 

 with the sexual reproductive process and lead to the formation of fruits 

 (matured ovary) and seeds (embryo, food endosperm, and seed coat). 

 Flowers may be composed of four sets of parts attached to the apex of the 

 stem and known as the receptacle (Fig. 71). Going from the outside 

 of the flower toward the center, the four parts are (1) sepals, (2) petals, 

 (3) stamens for the production of pollen, and (4) pistils for the recep- 

 tion of pollen and the production of ovules, the latter forming the mature 

 seeds. The sepals collectively constitute the calyx (Gr. kalyx, cup), while 

 the petals collectively constitute the corolla (L. corolla, crown). A com- 

 plete flower has all four sets of parts, while an incomplete flower has any 

 one of the four sets of parts lacking. Sepals and petals together con- 

 stitute the perianth (Gr. peri, around; anthos, flower). 



The odors of flowers are produced by the formation of chemical sub- 

 stances in special secreting cells, usually on the petals. The petals of 

 certain flowers have glands known as nectaries for the secretion of the 

 sweetish nectar, collected by insects. Flower colors usually result from 

 the presence of pigments known as anthocyanins (an tho -si' an in) (Gr. 

 anthos, flower; kyanos, dark blue) or carotenoids (kar' o ten oidz) (L. 

 car Ota, carrot or yellowish; Gr. eidos, form). The anthocyanins are 

 blue, red, and purple water-soluble pigments, while the carotenoids are 

 yellow, orange, and sometimes reddish pigments, 



A stamen consists usually of a stalklike filament and an enlarged 

 anther for the production of pollen. A pistil consists usually of an en- 

 larged, basal ovary within which seeds are formed, a slender style arising 

 from the ovary, and an enlarged, pollen-receiving stigma at the tip of the 

 style. The enlarged, ovary portion of the pistil is composed of one or 

 more carpels (Gr. karpos, fruit) within which are ovules from which seeds 

 are formed by fertilization. 



The angiosperms are widely distributed and numerous (approximately 

 200,000 species), so that it is impossible to study all of the many groups. 

 However, a detailed study of a few, well-selected representatives may 

 suffice for an orientation in the class as a whole. Consequently, Indian 

 corn [Zea mays) is selected because it is a large, common monocotyle- 

 donous type, the garden bean [Phaseolus) is chosen because it is a com- 

 mon dicotyledonous form, and the sunflower [H elianthus) because it is a 

 typical dicotyledonous type w ith a composite flower. 



