CROSS POLLINATION IN ENTOMOPIIILOUS FLOWERS 75 



dantly. The stigmas of insect-i)()llinat('d flowers as compared to 

 those of wiiid-])ollinated flowers are snuUl and ineoiisjiicuous. 



Inseet-}X)lliiiated flowers are further eliaraeterized l)y showiness. 

 Tliis may he (hie either to eoh)r, size, form or arrangement, or to 

 two or more of these features combined. In many cases, also, they 

 are characterized hy fragrance or by the production of nectar, or 

 both. 



47. Cross Pollination in Entomophilous Flowers.— The majority 

 of insect-polHnated flowers exhibit dichogamy to a greater or less 

 degree. In some cases the dichogamy is complete, that is, the stig- 

 mas are not receptive at any time when the stamens of the same 

 flower are discharging pollen (Fig. 29). ]\Iore often, however, there 

 is some overlapping so that, if cross pollination has failed to occur, 

 close or self-pollination is possible. Usually the stamens do not all 

 mature at the same time and the stigmas are receptive when either 

 the oldest or the youngest stamens are mature, depending upon 

 whether the species is characterized by protandry or protogyny. 



A less common but very interesting phenomenon facilitating cross 

 pollination is that known as heterostyly. In various species of 

 primrose {Primula), for example, some individuals produce long- 

 styled flowers and others short-stvled flow^ers. In the lons-stvled 

 flowers the five stamens, which are attached to the corolla, are 

 located half way down the corolla tube and the style is the same 

 length as the corolla tube so that the stigma is near its upper end. 

 In the short-styled flowers, on the other hand, the stamens are placed 

 at the upper end of the corolla tube and the style is only one-half the 

 length of the tube. When an insect visits a long-styled flower, pollen 

 from the low anthers is deposited on the anterior part of the body and 

 during subsequent visits to other flowers this pollen may come in 

 contact with the stigma of a short-styled flower. At the same time, 

 while the short-stj'led flower is being pollinated, pollen from the high 

 anthers is deposited on the posterior part of the insect's body and 

 this may later be rubbed onto the stigma of a long-styled flower, 

 While this morphological phenomenon of heterostyly in itself makes 

 cross pollination rather certain it is still further assured by the fact 

 that pollen from low anthers will not germinate readily on the stigma 

 of a long-styled pistil but will germinate readily on the stigma of a 

 short-styled pistil. On the other hand, pollen from high anthers W'ill 

 germinate readily only on the stigmas of long-styled pistils. 



Heterostyly is still further developed in the spiked loosestrife 

 (Lythrum salicaria) which has flowers of three types: long-styled, 



