180 



THE DICTIONARY OF GARDENING, 



Pollination continued. 



Fig. 217). They very often have special structures, e.g., 

 epurs or other modifications of parts, to form or to store 

 up nectar. They also possess a pleasant scent and 

 attract numerous insect visitors by the varied induce- 

 ments they offer. Some insects (e.g., Bees) also visit 

 flowers to eat or to collect pollen, or to carry it away as 

 food for their young progeny. Whatever the reason of 



Fio 217 EXPANDED BLOSSOM OF PEA c, Vexillum ; al, Al, with 

 Carina between. 



the visit, the insect generally becomes dusted with pollen, 

 which it transfers to the stigma of the next flower of 

 the same species that it enters. The pollen in entomo- 

 philous flowers is less abundant than in the anemophilous 

 ones ; and the grains very frequently bear ridges or 

 spines, so as to stick more readily to the insect, or they 

 are joined together in groups of four or more, as in 



1'iG. 218. POLLEN OF ORCHID. 



A, Pollen Masses, &c.po, Pollinia ; c, Caudicle ; vd, Viscid Disk ; 

 vg, Viscid Globe ; r, Rostellum ; Ir, Lip of Rostellum. B, 

 Pollen Granules (much magnified), held in packets by thin, 

 elastic threads. 



Heaths and Orchids (see Fig. 218). The masses are 

 furnished, in Orchids and a few other plants, with 

 special contrivances to favour adhesion to the insect's 

 body, and afterwards to place them in the best posi- 

 tion to touch the stigma of the flower next visited 

 (see Orchid Fertilisation). The stamens are usually 

 inclosed in, or are not longer than, the perianth, 

 and the anthers burst in such a way as to let free the 

 pollen in the position most likely to insure its being 

 dusted on to the insect. The stigma or stigmas do not 

 often project beyond the perianth, and are generally 

 small and rounded, or linear, down one side of the style. 

 The surface is usually covered with a layer of erect 

 cells, which secrete a viscid fluid, and in this the pollen 

 grains are caught when any part of an insect's body 

 dusted with them touches the stigmatic surface. The 

 pollen grains absorb nourishment from this fluid, and 

 are stimulated to emit pollen tubes between the cells of 

 the stigma and down the tissues of the style to the 

 ovules, to fertilise them. 



Both anemophilous and entomophilous flowers are 

 adapted to secure cross-fertilisation, or " allogamy " ; 

 while cleistogamous flowers, and a few others, are 

 adapted for self-fertilisation, or " autogamy." Darwin 

 and others have shown that allogamy secures the largest 

 production of healthy seeds, and that the seedlings are 

 stronger and healthier than when the stigmas are arti- 

 ficially fertilised with pollen from the same flower. The 



Pollination continued. 



disadvantages of allogamy are that it entails on the plant 

 a greater production of pollen, as by far the greater part 

 never reaches a stigma ; and, even with this, many 

 stigmas may remain unpollinated, and no seeds be pro- 

 duced in these flowers. Moreover, such flowers as have 

 been specially adapted for fertilisation by a certain kind, 

 or kinds, of insects, may, in absence of these agents, 

 remain nnpollinated and barren. This occurs with certain 

 greenhouse plants, which are fertile if Pollinated arti- 

 ficially, but, without human aid, remain barren, e.g., 

 various Orchids. Under Nectary and Orchid Ferti- 

 lisation several adaptations of flowers to benefit 

 by visits of insects will be found discussed, and only 

 one or two examples need here be added to those 

 referred to under " the above headings. By far the 

 most interesting examples of adaptations for Pollination 

 of the stigmas with pollen from another flower, are met 

 with among entomophilous flowers. Many of these are 

 suited to benefit by the visits of Beetles, Sawflies, and 

 other insects, which do not possess a long proboscis ; 

 hence, the nectar or pollen that attracts them is situated 

 almost on the surface, or, at least, is easily accessible, 



FIG. 219. STRAWBERRY PLANT IN FLOWER, showing the numerous 

 short Stamens, readily accessible to Insects. 



e.g., in the Strawberry (see Fig. 219). Such flowers may 

 have the pollen transferred from the anthers to the 

 stigma of the same flower ; but this is, in general, pre- 

 vented by the pollen and the stigmas not maturing 

 simultaneously (dichogamy), or by the direction in which 

 insects usually move on flowers, causing them to touch 

 the stigmas before they touch the pollen. Flowers of 

 this kind are often small individually, but are grouped 

 into conspicuous masses, e.g., in Umbelliferce and Com- 

 posite ; and, in snch cases, the outer flowers often differ 

 much from the inner in the inflorescence. This difference 

 is extreme in such plants as the Guelder Rose (Viburnum 

 Opulus), and in Hydrangea, where the outer flowers have 

 the perianth large and showy, but the sexual organs 

 abortive, and the inner flowers are small, but sexually 

 perfect, except in such garden varieties as have all the 

 flowers rendered showy and barren. 



But even among open and regular flowers examples 

 occur in which very perfect adaptations for cross-polli- 

 nation are present. Kalmia latifolia (see Fig. 220) may 

 be selected as an example. In this plant, the style rises 

 in the middle of the flower, bearing the small stigma on 

 its tip. There are ten stamens, curved as shown in the 

 figure, so that the anthers are situated each in a small 

 pouch in the corolla. In these pouches they remain till 

 the filaments are touched with a little force, and, if the 



