POLLINATING INSECTS 77 



short-styled, and intermediate-styled. Each of these three t.vpes of 

 flowers has two sets of stamens, the short-styled flowers having 

 intermediate and long stamens, the intermediate-styled flowers, 

 short and long stamens, and the long-styled flowers, short and inter- 

 mediate stamens. Thus pollen is readily transferred from any one 

 of these t^•pes to either of the other two. 



Flowers such as those of Lythntm, being of three forms, are said to 

 be trimorphic. Other plants with trimorphic flowers are the swamp 

 loosestrife {Decodon verticiUata) and the pickerel weed {Pontederia 

 cordata). 



A phenomenon that is well-known among fruit-growers is that of 

 self-sterility. ]Many of the common varieties of fruits, such as grapes, 

 peaches, plums, cherries, pears and apples are either partly or wholly 

 self-sterile, which means that they require pollen from some other 

 variety in order to produce healthy fruits. For this reason it is not 

 ordinarily recommended that orchards be planted with a single 

 variety but that two or more varieties whose blooming seasons 

 coincide or o\erlap should be used. 



48. Pollinating Insects.— By far the greatest number of pollinat- 

 ing animals are insects and the most important group of insects 

 from this point of view^ is the Hymenoptera which includes the ants, 

 bees and wasps. The honey-bees and bumble-bees are in certain 

 respects the most efficient of all pollinating insects. This is partly 

 due to the fact that they work continuously from the beginning to 

 the end of the flowering season; partly to the fact that they visit 

 flowers for both pollen and nectar; and partly because of their re- 

 markable precision which leads them to visit large numbers of flowers 

 of the same species in rapid succession. A honey-bee, for example, 

 visits only one kind of flower on any collecting trip and usually on 

 any one day. It is for this reason that it is often possible to purchase 

 pure clover honey, basswood honey, or buckwheat honey. Further- 

 more, the bees have hairy legs that are well fitted for carrying pollen, 

 and their long, tongue-like probosces enable them to obtain nectar 

 from irregular, tubular, and partly closed corollas. There are a con- 

 siderable number of plants that can be pollinated only })y bees and 

 such plants are limited in their distribution to those regions that are 

 inhabited bv bees. 



Ants are of no importance as pollinating agents and wasps, as com- 

 pared with bees, are of minor importance, although there are some 

 flowers, notably those of the figs, that are pollinated only by wasps. 



Next to the Hymenoptera, in importance as pollinating agents, 



