ANGIOSPERMS. 405 



or the tube does not effect fertilisation ; normal fertilisation is produced only by pollen 

 from another flower of the same species. Such a self-sterile plant is Coijiialis cava, 

 according to Hildebrand, in which the pollen of a flower falling on the stigma of the 

 same flower is ineffectual, and only effects fertilisation when conveyed to a flower of 

 another plant of the same species. Another plant of this kind is Oncidtui?i aricrochilmii ; 

 according to Fritz Miiller' the pollen-masses and the stigmas of the same plant in 

 various species of Oncidhun have actually a poisonous and deadly effect on one another. 



One of the commonest and simplest means by which self-pollination is prevented 

 is dichogamy, that is the development of the two kinds of sporophyll in a hermaphrodite 

 flower at different times. Either the anthers discharge their pollen before the stigma 

 is fully formed and in a condition to receive it, in which ease the flower \s protandrous, 

 or the style is developed before the anthers, and the stigma is dusted before the pollen 

 of the same flower has reached maturity and is discharged, and then the flower is 

 protogy notes. The stigmas of protogynous flowers can therefore only be dusted with 

 pollen from older flowers, those of protandrous flowers only with that of younger ones. 

 The Campanulaceae, Compositae, and Umbelliferae are protandrous. Plantago and 

 Aristolochia, etc. are protogynous. 



A further method for securing the mutual fertilisation of different plants of the same 

 species is heterogony ijieterostyly). In this case individuals of the same species differ in 

 respect to their sporophylls ; one has only flowers with a long style and short filaments 

 and with the stigma therefore placed high and the anthers low in the flower, while another 

 has flowers with the stigma low down in the flower and the anthers placed high, with a 

 short style therefore and long filaments. In this case then we have long-styled and short- 

 styled flowers on different individuals of the same species, as in Liniim peremie, P/itnu/a 

 sinensis, and others of the Primulaceae. But there are also plants, as for instance 

 Lythruni Salicaria and many species of Oxalis, in which the flowers of different indivi- 

 duals have sporophylls with three degrees of relative length ; beside the long-styled and 

 the short-styled form of flower, there is another form with styles of medium length. In 

 these cases of heterogony Darwin and Hildebrand have proved that fertilisation is only 

 possible {Linum perenne), or at any rate has the best result, when the pollen of the 

 long-st)ded flower is conveyed to the short-styled stigma, and that of the short-styled 

 flower to the long-styled stigma of another plant. Where there are three different 

 lengths of styles, fertilisation, by extension of the same rule, does best when the pollen 

 is conveyed to the stigma of another flower which stands at the same height as the 

 anther from which the pollen comes. 



The conveyance of the pollen from one flower to another is effected either by the 

 wind, as in the Cupuliferae, Urticaceae, Potamogeton, etc., or by insects ; less frequently 

 by birds, as in Colibris of the Marcgraviaceae, or by snails, as in some Aroideae. 

 Very various are the means by which insects are induced to visit flowers ; striking 

 colours in the perianth, small secretion of nectar, are among the number ; a detailed 

 description will be found in the writings of H. Mijller cited on the preceding page. 



Inflorescences. It is rare in Angiosperms for the flowers to appear singly on the 

 summit of the primary shoot or in the axils of the foliage-leaves ; it much more 

 frequently happens that peculiarly developed branch-systems are formed in these two 

 positions bearing flowers usually in great profusion ; these systems, which are known as 

 inflorescences, are distinguished by their collective form from the rest of the plant, and 

 in polycarpic plants are even thrown off after the ripening of the fruit. Their habit 

 depends not merely on the number, form and size of the flowers which they bear, but 

 also on the length and thickness of the component branches, and on the degree of 

 development of the leaves from the axils of which the branches spring; these are 

 usually much more simple in form and smaller than the foliage-leaves, and not unfre- 

 quently coloured (i.e. not green), or sometimes without colour. They are known by 



Bot. Ztg. 1868, p. 114. 



