REPRODUCTION AND DISPERSAL 



8S7 



and includes the asters, goldenrods, and sunflowers (fig. 1173). Perfect monocliny 

 is confined essentially to the second and to a part of the first group, so that a greater 

 or less amount of dicliny characterizes the majority of this great family. In the 

 third or ray-flowered group there are three common conditions : (i) that in which 

 all flowers are seed-producing, the disk flowers being monoclinous and the ray 

 flowers pistillate (as in Aster); (2) that in which the disk 

 flowers are monoclinous and the ray flowers reduced essen- 

 tially to corollas (as in Helianthus); and (3) that in which 

 the disk flowers are monoclinous but with the pistils sterile, 

 while the ray flowers are pistillate (as in Polymnia); Sil- 

 phium belongs in the last group, but it seems to have pro- 

 gressed still more toward dicliny, since the styles of the disk 

 flowers do not even fork into stigmas. 



Composites without ray flowers show as much diversity 

 as do the ray-flowered forms, though the latter are much 

 more numerous. Some forms have all flowers alike and 

 monoclinous (as in Eupatorium). In Artemisia the heads 

 in some species consist of monoclinous and pistillate flowers, 

 while in other species they consist of monoclinous but sterile 

 disk flowers and of pistillate marginal flowers. Iva exhibits 

 monoecious dicliny, the inner flowers being staminate and the 

 outer pistillate. Ambrosia and Xanthium also are monoe- 

 cious, but the two kinds of flowers are in separate heads. 

 The evolution of dioecism from monoecism appears to be 

 illustrated by Petasiies, for though all heads have monocli- 

 nous but sterile central flowers and pistillate marginal 

 flowers, some plants have heads with many staminate and 

 few pistillate flowers, while other plants exhibit the reverse 

 condition. Gnaphalium alpinum is essentially dioecious, 

 since in some plants the stamens do not shed pollen, while in 

 others the pistils are sterile. Complete dioecism is illus- 

 trated by the related Antennaria. Some composites, notably 

 Ambrosia, are wind-pollinated, as well as diclinous. The 

 possible significance of the remarkable floral diversity of 

 the Compositae will be considered elsewhere (p. 877). 



The protection of flowers from deleterious insects. — Deleterious kinds of insects. 

 — Crawling insects, such as the ants, are disadvantageous floral visitors, since the 

 pollen they carry is likely to be brushed off as they crawl from flower to flower. 

 Even among the flying insects, where such pollen losses are reduced to a minimum, 

 those that fly about in a haphazard manner, visiting various plant species in succes- 

 sion, are far less beneficial than are such insects as the bees which on any given 

 day visit individuals of the same plant species with consistency. It has often been 

 supposed that various floral features are highly advantageous because they exclude 

 certain insects, but the evidence for this view in many cases is more imaginary than 

 real. In any case, it is not to be supposed that the development of such features 

 has had any relation to deleterious insects; so far as they have value in this connec- 

 tion, it must be regarded as purely incidental. 



Fig. 1182. — A 

 flower from a dande- 

 lion head (Taraxacum 

 officinale), illustrating 

 epigyny ; note the 

 achene (0), the capil- 

 lary pappus {p) rep- 

 resenting the calyx, 

 the strap-shaped, five- 

 toothed, sympetalous 

 corolla {c), the tubular 

 column of syngene- 

 sious anthers (o) sur- 

 rounding the basal 

 portion of the style 

 (t), and the two re- 

 curved stigmas {g). 



