340 AUTOGAMY. 



itself again away from the axial position. All these movements are performed as 

 a general rule with great rapidity. In Alyssum calycinum they take a few hours, 

 in Draba verna the interval between morning and evening. 



Some annual species of the Wood-sorrel genus such as Oxalis stricta, for 

 example have five short and five long stamens in each flower. The anthers of the 

 latter are of the same height as the stigmas, but begin by being at a little distance 

 from them, so that at this period cross-fertilization may be effected by insects 

 alighting upon the stigmas. Within the space of a few hours, however, the longer 

 stamens incline towards the stigmas and deposit their pollen upon them. Here, 

 again, the pollen of the five shorter stamens does not reach the stigma in the same 

 flower, but is devoted to cross-fertilization. As in these annual species of Wood- 

 sorrel, so also in the majority of species of St. John's- wort (Hypericum), the unequal 

 length of the stamens, combined with the fact of the non-simultaneity of sexual 

 maturity (dichogamy), is of advantage to the plant in that it ensures that auto- 

 gamy shall take place just before the flower fades, whilst permitting at an earlier 

 period cross- fertilization by the pollen of other flowers through the agency of insects. 

 In Hyperiown perforatum, which may be chosen as our example, the pistil is encom- 

 passed by a number of filiform filaments of unequal lengths, so arranged that the 

 longest are in close proximity to the central pistil and the shortest near the peri- 

 phery of the flower. The anthers do not liberate their pollen simultaneously, but 

 one group does so after another. Dehiscence takes place first in the short stamens, 

 next in those of median size, and last of all in the long ones. As soon as an anther 

 opens, the filament bearing it inclines inwards, and thus one after another, the 

 short, median, and long filaments stand up and approach the middle of the flower. 

 It being only the anthers of the longest stamens that are on the same level as the 

 stigmas, autogamy cannot occur till quite at the last, not long before the flower 

 withers. 



The widely-distributed Star of Bethlehem (Orniihogalum umbellatum) exhibits 

 on the opening of the flower six stamens arranged in two whorls with three in each 

 whorl. The stamens of the inner whorl are the longer, and their anthers dehisce 

 a day sooner than the others. All six filaments look erect in the newly-opened 

 flower, but are really sloped slightly outwards, with the result that the anthers are 

 separated by an interval of about 3 mm. from the small stigma in the middle of 

 the flower. So long as they maintain this position the pollen cannot be directly 

 deposited upon the stigma, and the aim of the flower at this stage is to secure cross- 

 fertilization through the intervention of honey-seeking insects. Towards the end 

 of the flower's period of bloom both the long and the short stamens approach the 

 centre, but only the anthers of the three shorter ones are applied to the stigma and 

 besmear it with pollen; the other three do not come into contact with the stigma 

 owing to their standing at a higher level. Thus the Star of Bethlehem differs from 

 the cases previously described in that it devotes the pollen of the shorter stamens 

 to autogamy and those of the longer stamens to heterogamy (cross-pollination). 

 This is partly due to the circumstance that the anthers of the three longer stamens 



