IS 



1274 A TEXTBOOK OF THEORETICAL BOTANY 



common in Cruciferae, Caryophyllaceae and a variety of other families, 

 l^he opposite case of elongation of the style occurs in Epimediiim. The 

 flowers are more or less pendent and the valves of the open anthers, loaded 

 with pollen, are below the level of the stigma, which is only brought into 

 contact with them by growth of the style. 



Movements of elongation are often associated with bending movements, 

 particularly in the style, which by such means places the stigma among the 

 anthers. This is to be seen in most of the Rhinanthaceae and also notably 

 in Malva. Bending movements, pure and simple, may also affect the style, 

 as in some Labiatae, but are more often seen among stamens, which either 

 bend in towards the stigma {Lepidhini, Paris, Lysimachia), or arch them- 

 selves over the stigma and drop pollen directly on it (many Umbelliferae). 

 In some of the Cactaceae, e.g., Cereiis, the stigma is at first thrust out well 

 beyond reach of the pollen from the numerous stamens, but later is re- 

 tracted by shrinkage of the long style, so that it is finally surrounded by 

 pollen-laden anthers. Bending of the stigma is common among the Com- 

 positae. The long stigmatic lobes, which are receptive only on the upper 

 surface, curl over so that this surface touches the style or the top of the 

 anther tube, below the stigma, and thus picks up any remaining pollen. 



Some actinomorphic flowers which are freely exposed may be auto- 

 gamously pollinated by rain. A large drop collects in the flower and on this 

 drop the pollen floats and finds its way to the stigmas. Hagerup has observed 

 this in various species of Ranunculus, Caltha and Narthechirn. 



Lastly there are the instances of the corolla aiding in autogamy, which 

 are often striking. Where the stamens are attached to the wall of a floral 

 tube, the anthers may be brought against the stigma by the closing of the 

 flower at the end of anthesis. The infolding of the petals in a sleep-move- 

 ment may cause pollination, if pollen has previously been shed upon them. 

 A good example in the Papaveraceae is Argemone mexicana. In flowers with 

 tubular corollas, even if the tubular portion is very short, the abscission of 

 the corolla as a whole allows it to slide forward along the stamen filaments, 

 thus forcing the anthers into contact with the stigma. This occurs in 

 several well-known plants, such as Rhododendron, Anagal/is, and Digitalis. 

 Even without abscission, the elongation of the corolla tube may carry pollen 

 held on its surface upwards to the level of the stigmas. 



These are only a few of the multitudinous devices which have been 

 noted, but they make clear the general importance of the process among the 

 flowering plants. 



We have already dealt with the probable origin of dioecism in the pre- 

 vious chapter (p. 1 131) and have shown reason to believe that the unisexual 

 condition is secondary and derived from the hermaphrodite condition. As 

 an example of the complexity which may exist in flowers in a transitional 

 state, we may take the Trailing Arbutus, Epigaea repens. The flowers may 

 be divided into two groups. The first group has large, well-developed, 

 radiate stigmas, which are glutinous and retentive of pollen. The second 

 group has small, erect, dry stigmas, on which pollen does not germinate. In 



