854 ECOLOGY 



pollen and stigma are to play a part in pollination; this is an apparent disadvan- 

 tage as compared with horaogamous flowers. 



Heterostyly. — A highly specialized condition that opposes close 

 pollination and favors cross pollination is that known as heterostyly, in 

 which the stigmas and the anthers in different flowers occupy different 

 positions. Most such flowers are actinomorphic, and they are illustrated 

 by the primrose, flax, forget-me-not, and bluets. In the primrose some 

 plants have flowers with long styles, the stamens being attached toward 

 the base of the corolla tube, while other plants have flowers with short 



styles and with the sta- 

 mens attached toward the 

 upper part of the corolla 

 tube (figs. 1180, 1181). 

 In Lythrum Salicaria there 

 are three kinds of flowers, 

 one with long styles and in- 

 termediate and short sta- 

 mens, another with short 

 styles and intermediate 

 and long stamens, and a 

 third with intermediate 

 styles and long and short 

 stamens. The same part 

 of the insect that comes in 

 contact with the lower sta- 

 mens will touch the stigma of a short-styled flower, while pollen from 

 the upper stamens will come in contact with the stigma of a long-styled 

 flower, thus insuring cross pollination. 



Commonly the upper stamens of heterostyled flowers have large pollen grains 

 corresponding to the large long-haired stigmas of the long-styled flowers, while 

 the lower stamens have small grains corresponding to the small smooth stigmas of 

 the short-styled flowers; the corollas and other organs also may differ considerably. 

 Some investigators regard the large size of the pollen grams of the upper stamens 

 as advantageous, since their pollen tubes have to traverse a greater distance upon 

 germination; this view, which is doubtful a priori on account of their parasitic 

 nourishment, has been experimentally disproven. 



Impotent and prepotent pollen. — • So far as the prevention of close 

 pollination is concerned, the most specialized flowers are those in which 

 the pollen of a given flower is impotent (i.e. unable to initiate seed produc- 



1180 



Figs. 1180, 1181. — Longitudinal sections through 

 flowers of the Chinese primrose {Primula sinensis), 

 illustrating heterostyly: 11 80, a flower with a long 

 style (t) and with stamens (s) inserted near the base 

 of the corolla tube (c'); 1181, a flower with a. short 

 style (t) and with stamens (s) inserted at the median 

 part of the corolla tube (c'); that part of a visiting 

 insect which strikes the anthers of one flower will be 

 hkely to strike the stigma of the other, thus effecting 

 cross pollination; note that the corolla tube of 1181 

 is dilated where the stamens are inserted; these 

 flowers illustrate perigyny; c, calyx; 0, ovary. 



