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Show ~374 MEANS OF CROSS-FERTILISATION. CHAP. X. visit and suck a greater number of flowers within a given time than would otherwise be possible; and thus there will be a better chance of fertilisation by pollen brought from a distinct plant, and this we know is of paramount iin portance. The odours emitted by flowers attract insects, as I have observed in the case of plants covered by a muslin 'net. Nageli affixed arti:fici~l flowers to branches, scenting some with essential oils and leaving others unscented; and insects were attracted to the former in an unmistakable manner.* Not a few flowers are both conspicuous and odoriferous. Of all colours, white is the prevailing one; and of white flowers a considerably larger proportion smell sweetly than of any other colour, namely, 14 · 6 per cent. ; of red, only 8 · 2 per cent. are odoriferous. t The fact of a larger proportion of white flowers smelling sweetly may depend in part on those which are fertilised by moths requiring the double aid of conspicuousness in the dusk and of odour. So great is the econon1y of nature, that most flowers which are fertilised by crepuscular or nocturnal insects emit their odour chiefly or exclusively in the evening. Some flowers, however, which are highly odoriferous depend solely on this quality for their fertilisation, such as the night-flowering stock (Hesperis) and some species of Daphne ; and these present the rare case of flowers which are fertilised by insects being obscurely coloured. The storage of a supply of nectar in a protected place is manifestly connected with the visits of insects. So is the position which the stamens and pistils occupy, * 'Enstehung, &c., dcr Naturhist. Art.' l 865, p. 23. t The colours and odours of the Howers of 4200 species have been tabulated by Landgrahe, and by Sclri.ibler and K~il~ler. I l1ave· not seen t' he n· · Ol'lo<n nal works but a very full abstra~t is o·ivc~n in Loudon's 'Gardener 8 M~g.' vol. xiii. 1837, P· 367. CHAP. X. MEANS OF CROSS-FERTILISATION. 375 eit~er permanently or at the proper period through then o~n movements; for when mature they invariably stand In the pathway leading to the nectary. The s~1ape. of the nectary and of the adjoining parts are likewise related to the particular kinds of insects which habitually visit the flowers; this has been well shown by. H. Miille~ by ~i~ comparison of lowland species whiCh .are chiefly VISited by bees, with alpine species belonging to the same genera which are visited by b~tterflie~. * Flowers m~y also be adapted to certain ~1nds of Insects, by secreting nectar particularly attractive to them, and unattractive to other kinds; of which fact Epipaetis lat~folia offers the most striking instance known to me, as i~ is visited exclusively by wasps. Structures also exist, such as the hairs within the corolla of the f~xglove (Digitalis), which apparently serve to exclude Insects that are not well fitted to bring pollen from one flower to another.t I need say nothing . here of the endless contrivances, such as the viscid glands attached to the pollen-masses of the Orchidere and Asclepiadre, or the viscid or roughened state of the. pollen-grain~ of many plants, or the irritability of then stamens which move when touched by insects, &c., -as all these contrivances evidently favour or ensure cross-fertilisation. All ordinary flowers are so far open that insects can force a~ entrance into them, notwithstanding that som~'. hke the Snapdragon (Antirrhinum), various Pap1honaceous and Fumariaceous fJ owers, are in appearance closed. It cannot be maintained that their op~nness is necessary for fertility, as cleistogene flowers whwh are permanently closed yield a full complement of seeds. Pollen contains much nitrogen and phos- * 'Nature,' 1874, p. 110; 1875, t Belt, ''rhe Naturalist in Ni-p. 190; 1876, pp. 210, 289. caragua,' 187-!, p. 132. |