340 GENERAL BOTANY 



corresponding parts of the flower of the angiosperms and the 

 strobili of the gymnosperms are at once made apparent. 



The receptacle of the flower evidently corresponds to the axis 

 of a strobilus, although it is greatly shortened and somewhat 

 flattened at its apex. The stamens correspond to the microsporo- 

 phylls and microsporangia of the spruce strobilus, the filament 

 representing a highly modified, slender microsporophyll, and the 

 anther sacs representing microsporangia borne at the apex of the 

 sporophyll. A single pistil of the marsh marigold flower corre- 

 sponds to a single megasporophyll on the female strobilus of the 

 spruce, in which the edges have folded in and united so as to 

 inclose the megasporangia, or ovules, in a cavity called in the 

 angiosperm flower the ovary cavity. 



The perianth is evidently a new structure which functions to 

 protect the essential organs, the stamens and pistils, during their 

 development. We have learned that in highly organized flowers 

 like the locust and the bean the perianth may also serve an 

 important function in securing cross-pollination by insects. The 

 perianth in some flowers undoubtedly represents transformed 

 sporophylls at the base of a strobiluslike flower, while in other 

 instances it is apparently formed from ordinary green leaves 

 below the sporophylls. We may conclude, therefore, that the 

 angiosperm flower, represented by the flower of the marsh mari- 

 gold, is a highly modified strobilus, in which many changes have 

 taken place during its long course of evolution, including the 

 shortening of the axis, or receptacle, and the transformation of 

 simple sporophylls and sporangia into stamens and pistils and 

 of certain sporophylls, or green leaves, into the parts of the 

 perianth, namely, the calyx and corolla. 



The evolution of the sporophylls and sporangia of the angio- 

 sperm flower will be more fully understood if a further com- 

 parison is made between these structures and the corresponding 

 structures in the other vascular plants already studied. In 

 Fig. 201 the microsporophylls and sporangia of Selaginella^ spruce, 

 and marsh marigold are compared with a portion of the sporo- 

 phyll, or leaf, of Adiantum. From this figure it will be seen 

 that the sporophyll has become gradually reduced in size from 



