REPRODUCTION AND DISPERSAL 



875 



become thick protective structures of considerable value to the embryos, 

 but often during the earlier stages of growth some protection from desic- 

 cation and other dangers is afforded by special structures or habits. 

 After anthesis the calyx may close about the developing fruits (as in 

 Physalis), appearing much as in the bud, thus once more serving as an 

 organ of protection, as well as continuing to manufacture food. In 

 the composites the protective and synthetic activity of the calyx is re- 

 placed by that of the involucre. Many fruits which 

 are edible when ripe, during immaturity are more 

 or less protected from predatory animals by un- 

 pleasant flavors, hardness, and spinescence (as in 

 Opuntia and Ribes), and perhaps by inconspicu- 

 ousness, since their green color is similar to that of 

 the leaves. 



In many plants, especially in hydrophytes, the 

 developing fruit-stalks exhibit striking 'growth cur- 

 vatures. In most cases the previously erect stalk 

 recurves (as in Pellandra and Nymphaea), causing 

 the downward orientation and consequent submer- 

 gence of the fruit. Some land plants show similar 

 reactions, notably Phry-ma (fig. 1198), whose fruits 

 become strongly reflexed, whence the common 

 name, lopseed; in the peanut, stalks that are erect 

 or ascending until anthesis, later recurve and force 

 the developing nut into the ground. Probably in 

 all of the above cases pedicels or peduncles orig- 

 inally apogeotropic become progeotropic after an- 

 thesis, but no explanation of such peculiar behavior 

 has been given, nor is there any obvious advantage, except perhaps a 

 slight one, in connection with planting (p. 929). 



The origin of floral structures. The flower is the most complicated 

 of plant structures, and the organs concerned directly or indirectly with 

 pollination form the most complicated part of the flower. An adequate 

 theory of flower structure, however, must explain not alone this complex- 

 ity, but also the evolution of the mouth parts of the flower-visiting in- 

 sects (notably those of the bees, butterflies, and moths), which appear 

 to be so obviously related to the flowers. Capping all, and most diffi- 

 cult of all to explain, are the cases of obligate reciprocal- symbiosis, of 

 which the fig and the yucca are the most remarkable. 



FIG. 1 198. A flow- 

 ering spike of the lop- 

 seed (Phryma Lepto- 

 stachya) ; at anthesis 

 the flowers (/) with 

 their bilabiate corollas 

 (c) are horizontal, but 

 subsequent epinastic 

 growth causes the 

 fruits (/') to become 

 strongly reflexed. 



