REPRODUCTION AND DISPERSAL 839 



clinous (figs. 1136, 1137), though a few are diclinous; for example, the 

 willows are dioecious, and many composites are monoecious. Dicliny 

 has been thought to be advantageous in wind-pollinated flowers because 

 it increases the probability of cross pollination; however this may be, 

 monocliny would seem to have a distinct advantage in insect-pollinated 

 flowers in that it makes possible double the amount of pollination for 

 a given number of insect visits. Furthermore, pollen-gathering insects 

 would not visit pistillate flowers, and nectar-gathering insects would visit 

 both pistillate and staminate flowers only in case each were nectar- 

 bearing, thus involving two nectaries in one act of pollination. 



Pollen. The stamens of insect-pollinated flowers rarely are promi- 

 nently exserted and the filaments often are short; also the inflorescences 

 are relatively inflexible in the wind. The pollen, instead of being dry 

 and powdery, commonly is adhesive through the possession of spines or of 

 other protuberances (figs. 1156, 1157), or through the presence of viscid 

 substances (as in Oenothera), so that the grains often cohere in masses. 

 The shape of the grains is more likely to be elliptical than spherical, the 

 latter shape being especially characteristic of the grains in wind-polli- 

 nated flowers. Such pollen grains are not easily blown about by the 

 wind, and they adhere readily to visiting insects and to stigmatic surfaces. 

 In species with wide-open flowers, which therefore are exposed to insects 

 of all kinds, including pollen-gathering insects, the pollen often is almost 

 as abundant as in wind-pollinated species; sometimes also the stamens 

 in such flowers are very numerous (as in the roses and buttercups). In 

 tubular or otherwise partly closed flowers, where the stamens are con- 

 cealed, the latter commonly are few in number and the pollen is relatively 

 sparse (as in the phloxes and mints). As a rule, the stigmas are smaller 

 and otherwise less conspicuous than in wind-pollinated flowers. 



Features supposed to be attractive to insects. The most noticeable 

 single feature of insect-pollinated flowers is their showiness, which is due 

 to the color of the flowers, or to their size, position, or arrangement. 

 Many insect-pollinated flowers are fragrant, and many also possess 

 nectar. It is rare that a flower which is pollinated regularly by insects 

 is neither showy, fragrant, nor nectar-producing, and some insect-polli- 

 nated flowers have all these features. 



Most insect-pollinated plants north of the tropics are of low stature, but in warm 

 countries many trees have insect-pollinated flowers. An odd phenomenon, com- 

 monest in the humid tropics (but characteristic also of our northern redbud, Cercis 

 canadensis), is cauliflory (i.e. stem flowering), the tree trunks often being covered 



