BEPRODTJCT1ON IN PLANTS 



107 



Fig. 57. A microspore and 

 megaspore of a 'higher Fern", 

 Selaginella, magnified and drawn 

 to the same scale. (From 

 Coulter.) 



4. Flowering Plants 

 Passing to the Flowering Plants, 

 we find that these are heterosporous 

 sporophytes, and the FLOWER rep- 

 resents a greatly modified stem 

 (branch), the leaves of which are 

 specialized as sporophylls and ac- 

 cessory structures. In order to 

 make this clear it is necessary to 

 review the structure of a typical 

 flower. (Figs. 40, 58.) 



A complete flower consists of four whorls of modified 

 leaves. These arise near together at the tip of a PEDUNCLE, 

 representing the floral branch, which connects the flower 

 proper with the main tissue systems of the plant as a whole. 

 The outer and lower circle of leaves 

 (CALYX) is composed of several 

 parts (SEPALS) which usually are 

 green and retain a leaf-like appear- 

 ance. Just within and above the 

 calyx is the second circle (COROLLA) 

 formed of larger leaves (PETALS) 

 which are usually brightly colored. 

 The calyx and corolla together form 

 the PERIANTH, or floral envelope 

 which surrounds the primary floral 

 organs, the STAMENS and CARPELS. 



The stamens represent the third 

 circle of leaves, but are so highly 

 modified that their leaf origin is not 

 ^mediately apparent. Each con- 



&, calyx; c. corolla; d, stamens; gists of a slender FILAMENT at the 

 e, pistil formed of fused carpels. . . . . . 



(Modified from Gager.) apex of which is a small case known 



