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AUTOGAMY IN CLEISTOGAMIC FLOWERS. 391 



temperature be not too low. The stigmatic tissue becomes receptive; the anthers 

 attain to maturity, dehi.sce and liberate their pollen notwithstanding that no ray of 

 sunshine penetrates the clouds and that rain falls continuously. In such circum- 

 stances the mouth of the flower is not opened; autogamy takes place in the closed 

 flower, and all the adjustments evolved with the object of ensuring cross-fertilization 

 are ineffectual. This is the case, for instance, in the following: — Alsine rubra, 

 Anagallis phoenicea, Arahis coerulea, Azalea procumbens, Calandrinia corri'pressa, 

 Gentuncidus minimus, Drosera longifolia, Gagea lutea, Gentiana campestris, G. 

 glacialis, G. prostrata, Hypecoum pendulum, Hypericum humifusum, Lepidium 

 sativum, Montia fontana, Oxalis corniculata, 0. stricta, Polycaiyon tetraphyllum, 

 Portulaca oleracea, Sagina saxatilis, Silene noctiflora, Sisyrinchium anceps, Sptr- 

 gida arvensis, Stellera Passerina, Veronica alpina, V. bellidioides and V. Chamce- 

 drys — plants which grow in widely different habitats, but which all have the com- 

 mon property that their flowers open for but a short period, if at all. In plants 

 with long-lived flowers it is of not uncommon occurrence for autogamy to be 

 j accomplished during a spell of wet weather, and for the petals to open subsequently 

 all the same, and so afford the possibility of the remains of the pollen being carried 

 away by insects. This phenomenon has often been observed, for example, in 

 I Rhododendron hirsutum, the Bog-bean {Menyantlies trifoliata), and the Greater 

 Dodder (Cuscuta EurojKca). 



There is also the case of such plants as Alisma natans, Illecebrum verticillatum, 



Limosella aquatica, Peplis Portula and Subularia aquatica, which live in pools or 



on the banks of ponds where the level of the water is variable. If the buds of 



j these plants are submerged at the time when they are about to open, they do not 



I unfold, and autogamy takes place in the closed flowers under water. It must be 



I observed that the water does not penetrate into the air-filled interior of the flowers, 



so that we have here the curious phenomenon that the transference of pollen to the 



1 stigma, though accomplished under water, is yet a case of pollination in the medium 



I of the air. 



j An allied phenomenon is exhibited by some of the Knotweeds (Polygonum 



I Hydropiper, P. minus, and P. mite). Isolated plants of an}^ of these species, in 



which all the flowering branches are exposed to the sunshine, and are both visible 



I and accessible to insects, unfold all their flowers; but, if hundreds of one species are 



I crowded close together, only a limited number of the flowers open their perianths. 



The flowers growing on the upright branches alone of such crowded plants unclose, 



i and receive insects' visits, whilst those which grow on the under, procumbent 



branches, and are consequently concealed and not easily reached by insects, remain 



shut. Nevertheless autogamy is effected wnth obvious success in these also. 



Plants of the kind just alluded to form a transition to those which normally 

 produce two kinds of flowers, viz.: some which open and are adapted to cross- 

 fertihzation through insect-agency, and some which remain closed and exhibit 

 autogamy with great regularity. The latter have received the name of cleistogamic 

 (<cXet<rT05=that can be closed, -ydixos, marriage; flowers, and amongst them may be dis- 



