324 A TEXTBOOK OF BOTANY [On. VI, 10 



to indicate the general course of that evolution. Thus we 

 can trace the anthers and pollen grains back without any 

 serious break to SPORANGIA (or SPORE CASES) and SPORES 

 (the kind called microsporangia and microspores) of the 

 highest flowerless plants, each anther being a composite 

 microsporangium and each pollen grain a microspore. 

 We can trace the ovules back in the same way to MEGA- 

 SPORANGIA and MEGASPORES (Fig. 223), each nucellus being 

 a megasporangium, and the embryo sac a megaspore, while 

 the integuments are a special new outgrowth from the 

 stalk of the sporangium. We can, however, trace these 

 parts still farther back to an origin in a single kind of sporan- 



FIG. 222. Diagrams to illustrate, in cross section, the various ways in 

 which ' carpels, here five in number, unite to form compound pistils and 

 placentae. 



First, carpels all separate; second, united like Fig. 221, giving parietal 

 placentae; third, infolded to the center, like the first but grown together, 

 giving central placentae ; fourth, like the third, but with the partitions 

 wanting, giving free central placenta. 



gium and spores, such as we find in the Ferns, where they 

 occur in the brown sori, or " fruit dots," on the backs of the 

 fronds (Fig. 224), and we can even trace them, if we choose, 

 back into the Algae. Thus we see that pollen grains with 

 the anthers, and embryo sacs with the ovules, are mor- 

 phologically equivalent to the spores and spore cases of the 

 lower plants, and are therefore far older than the other 

 parts of the flower. Hence a flower consists morpholog- 

 ically of stem, leaves, and sporangia with their spores. Or, 

 since the spores are the more important as well as the older 

 parts, we may say that morphologically a flower consists of 

 spores together with stem and leaves specialized to aid in 

 their reproductive function. 



