SECT. II ■ PHANEROGAMIA 495 



are so oriented that after tlie sporangia have opened the pollen can be readily 

 carried away by the wind. Examples are the catkins of the Oak, Birch, Alder, 

 Hazel, and Walnut ; the catkins of the last (Fig. 516) are especially long. The 

 male flowers of the Coniferae, the shedding of the pollen of which has been investi- 

 gated by GoEBEL, are similar. The pollen grains also have characteristic features. 

 They are light and smooth, and in some Conifers are even provided with two wing- 

 like sacs (Fig. 424 D) which enable them to remain suspended longer in the air. 

 Some Urticaceae (Pilca, Urtica) scatter the pollen on the opening of the elastically 

 stretched wall of the pollen-sac as a light cloud of dust. 



The female flowers are usually not brightly coloured and do not develop 

 nectaries. The stigmas, which catch the pollen, are strongly developed and pro- 

 vided with long feathery hairs, or their form is brush-like, pinnate or elongated, 

 and filamentous. In many Gymnosperms {e.g. Taxus) the macrosporangium 

 excretes a drop of fluid in which the pollen-grains are caught ; on drying up of 

 the drop the pollen is drawn down on to the tip of the nucellus. In other cases the 

 pollen grains glide down between the carpellary scales of the cones till they reach 

 the moist micropyles of the ovules and adhere to them. 



Lastly, the time of flowering is not without importance. The Elm flowers in 

 February and March long before its leaves develop, and the same holds for the 

 Hazel, Poplar, and Alder. In the Walnut, Oak, Beech, and Birch the flowers open 

 Avheu the first leaves are unfolding and flowering is over before the foliage is fully 

 expanded. Were this not so, much of the pollen would be intercepted by the 

 foliage leaves and even more pollen would need to be produced than has to be done 

 to ensure fertilisation. In the Conifers the foliage presents less difficulty, but here 

 the female cones are borne at the summit of the tree (Abies) or high up (Picea), 

 while the male flowers are developed on lower branches. The pollen grains are 

 shed in warm dry weather, and carried up in the sunshine by ascending currents 

 of air till they reach their destination on the female cones situated high above the 

 male flowers. 



Only a small number of Phanerogams make use of the agency of water for 

 effecting their pollination, and are, on that account, termed hydrophilous plants. 



The pollen of the submerged Zostcra exhibits certain peculiarities, distinctly 

 referable to the necessity of effecting fertilisation under water. It does not form 

 round grains, but in their place elongated thread-like filaments devoid of an exine, 

 which, as they have the same specific weight as the surrounding water, are easily 

 set in motion by the slightest currents, and are thus brought into contact with the 

 stigmas. Species of HalofMla which live in tropical seas behave in a similar 

 fashion. In the case of the submerged water plants, ValUsneria, Elodca, Hydrilla, 

 and Enalus, the pollination is accomplished on the surface of the water. Thus, 

 for example, the male flowers of ValUsneria, after separating from the parent 

 plant, rise to the surface of the water, where they open and float like little boats 

 to the female flowers ; these, by the elongation of their spirally coiled flower-stalks, 

 ascend, at the same time, to the surface of the water, only to become again sub- 

 mertred after fertilisation. 



'&^ 



The great majority of Phanerogams are dependent upon animals, 

 especially on insects, for the transference of their pollen. Plants 

 pollinated by the aid of insects are termed entomophilous. Since the 

 discovery of the relations between flowers and insects by Konrad 

 SPRKNGiiL, investigations into floral-o?cology have been so actively 

 carried on that one thinks first on this small department of the whole 



