328 THE CROSSING OF FLOWERS. 



which have not been so visited, for they would otherwise wither without being 

 pollinated. In this sense we may regard the staminate flowers of Polygonum 

 Bistorta as reserve flowers which, in case of absence of insect-visits, can come to the 

 rescue in the last extremity with their own pollen. 



Many plants related to the Bistort belonging to the Rhubarb and Dock genera 

 (Rheum, Rumex), and many species of the Meadow-rue {Thalictrum) belonging to 

 the Eanunculaceae, agree for the most part with the case just described. The her- 

 maphrodite flowers of the Rhubarb are protandrous. The anthers project, one after 

 the other, above the edge of the tiny bell-shaped perianth, where they open and 

 emit their floury pollen. This is easily shaken off by the least movement, and soon 

 afterwards the anthers tumble off' their filaments. At this time the three styles on 

 the top of the ovary are bent back, and the large, swollen, cauliflower-like stigmas are 

 so hidden at the base of the perianth that the pollen can gain no access to them. 

 Not until all the anthers have fallen off do the styles straighten and place their 

 succulent three-lobed stigmas in front of the edge of the perianth. Since the 

 development of the extensive inflorescences of the Rhubarb takes place only 

 gradually, one flower withering when another near it has just opened, the pollen 

 shaken from the anthers of the younger flowers usually falls on the stigmas of the 

 older ones. Sometimes the pseudo-hermaphrodite staminate flowers, which also occur 

 in the inflorescence of the Rhubarb, and which are the last to open, have to provide 

 the pollen for the adjoining hermaphrodite flowers, and after having performed 

 their task they fall off. The course of development in the Alpine Dock (Bumex 

 alpinus) gives rise to geitonogamy, but the process differs from that in the 

 Rhubarb, since the stigmas do not emerge from their hiding-place in the depths 

 of the perianth by the straightening of the style, but are rendered conspicuous by 

 the folding back of the perianth-leaves, whilst in several Meadow-rues (Thalictrum 

 alpinum, fostidum, and minus) the stigmas, which are at first concealed under the 

 petaline sepals, are exposed and rendered accessible to the pollen of neighbouring 

 flowers by the falling away of the sepals which cover them. 



These plants have floury pollen which, in the absence of wind, may fall verti- 

 cally on the stigmas of neighbouring flowers, but whose transport is usually effected 

 by breezes. They therefore afford a transition to such plants as have hermaphrodite 

 flowers in which geitonogamy is chiefly brought about by the wind, although it 

 may also result in the same way as in the Meadow-rues and the Docks and 

 Rhubarbs. These plants were mentioned when we were considering the inadvis- 

 ability of dividing plants into those which are respectively anemophilous and ento- 

 mophilous (see p. 129). These plants would belong to both classes; at first they are 

 insect-fertilized, and later on they are fertilized by the wind. The Mediterranean 

 Heath (Erica carnea), which grows in Alpine districts from the valley-floor almost 

 to the summits of limestone mountains, may be taken as the type of some two 

 hundred Ericaceae. This plant is much frequented by bees, and their visits are 

 the cause of manifold crossings, sometimes between the flowers of the same plant, 

 sometimes with other plants. In this plant, however, the crossing of neighbounng 



