856 ECOLOGY 



stigmas of different position in separate plants is quite as ineffective as 

 is close pollination, thus showing clearly that the cause of impotence is 

 not closeness of relationship, but something as yet unknown. A possible 

 advantage of this peculiar phenomenon is seen in the fact that the prog- 

 eny of individuals which are close pollinated, or of individuals where 

 there is cross pollination between anthers and stigmas of different posi- 

 tion, usually is made up of plants with but one kind of flower. If cross 

 pollination is advantageous (see p. 866), the combination of heterostyly 

 and impotence in own pollen would seem to be particularly advantageous, 

 since own pollen is scarcely likely to be deposited on a stigma, and if it 

 should chance to lodge there, it would not initiate seed production. 



Dicliny. Dicliny, which commonly is regarded as a primitive floral 

 feature, is more characteristic of wind-pollinated than of insect-pollinated 

 plants, but it is far more common in the latter than formerly was supposed, 

 and there is almost certain proof of a strong evolutionary tendency from 

 monocliny to dicliny, as in the figs and in many composites. Among 

 the diclinous insect-pollinated species that probably are primitive, the 

 best known are the willows, which are dioecious. Some investigators 

 doubt whether as many as half of the plants that appear to be monoclinous 

 are so in fact. A large number of species have both monoclinous and 

 diclinous flowers on the same or on different plants; the maples illus- 

 trate this condition, some of them (as the box elder) appearing to have 

 become completely dioecious. Certain cultivated varieties of the straw- 

 berry exhibit similar features. 



Asparagus appears to have become essentially dioecious, since the stamens of 

 some plants and the pistils of others appear to play no part in pollination. Many 

 species (as in the grape and the horse chestnut) have been found to possess impotent 

 pollen in some flowers, and non-receptive stigmas in others. Rhamnus lanceolata, a 

 heterostyled species, seems to be approaching dioecism, since the short-styled flowers 

 produce the most seed, while the long-styled flowers have but little pollen and that 

 small-grained. Many plants have organs occupying the position of stamens, which 

 now play no direct part in pollination, whatever may have been the case formerly ; 

 notable illustrations are the so-called sterile stamens of Parnassia (now nectar- 

 secreting organs) and of Pentstemon. 



Much the most significant tendency toward dicliny is seen in the composites, 

 which commonly are regarded as the highest family of plants. In this family there 

 are three common floral conditions, that in which all the flowers are actinomorphic 

 and inconspicuous (as in Eupatorium), that in which all the flowers have conspicu- 

 ous strap-shaped (ligulate), zygomorphic corollas (as in the dandelion, fig. 1182), 

 and that in which there are actinomorphic and inconspicuous disk flowers, sur- 

 rounded by petal-like zygomorphic ray flowers; the third group is much the largest 



