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Show 402 ANEMOPHILOUS PLANfS. CHAP. X.. Althouo·h it is probable that pollen was aboriginally the sole 0 attl·action to insects, and although m.any plants now exist whose ~owe1~s are frequented exclusively by pollen-devounng Ins?cts, yet. th~ great majority secrete nectar as the c~Ief .attraction. M~ny years ago I suggested that p:1manly the s~cchanne matter in nectar was excreted as a waste product of chemical chan <res in the sap; and that when the excretion happe;ed to occur within_ the envelop~s of a flower it was utilised for the Important object of cross-fertilisation, .being subsequently much increased in quantity and stored in various ways. This view is rendered probable by the leaves of some trees excreting, under certain climatic c.onditio.ns, ~ithout the aid of special glands, a sacchanne fiu1d, often called honey-dew. This is the case with the ]~aves of the lime; for although some authors have disputed the fact a most capable judge, Dr. Maxwell Master~, info'n ns me that, after having heard the discussiO. ns on this subject before tho Horticultural Society, he feels no doubt on this head. The leaves, as well as the cut stems, of the manna ash (Fraxinus ornus) ~ecrete in a like manner saccharine rna tter. t According to Treviranus, so do the upper surfaces of tho loaves of Oard·uus arctt'oides during hot weather. Many ana~ logous facts could be given.t There arc, however, a considerable number of plants which bear small glands§ on their leaves, petioles, phyllodia, stipulos, * Nectar was reg-arded by De C.mdollo and Dunal as an 'Xcret ion, as stuted by 1\Jartinot in' Annal des Sc. Nat.' 1872, tom. xiv. p. 211. t 'Gar·d. Chr0n.' 1876, p. 242. t Kurr, 'Untersuchungen iibrr 1lie Bed0ntung der N ektarien,' 1833, p. 115. § A laro·o number of cases are given by ~::-Delpino in the 'Bul.ldino J£ntomologico,' Anno v1. 1874. 'l'o these mny bo added those given in my text, as ~ell ns tllo excretion of saccharmo matter from the calyx of two species of Iris, and .from tho ~ra~~ tom of certain Crclmloro : sec h.ulr ' CHAP. X. SECRETION OF NEC'rAR. 403 bractem, or flower peduncles, or on the outside of their calyx, and these glands secrete minute drops of a sweet fluid, which is eagerly sought by sugar-loving insects such as ants, hive-bees, and wasps. In the case of the glands on the stipules of Vicia sati·va, the excretion manifestly depends on changes in the sap, consequent on the sun shining brightly; for I repeatedly observed that as soon as the sun was hidden behind clouds the secretion ceased, and the hive-bees left the field; but as soon· as the sun broke out again, they returned to their feast.* I have observed an analogous fact with the secretion of true nectar in the flowers of Lobelia erinus. Delpino, however, maintains that the power of secreting a sweet fluid by any extra-floral organ has been in every case specially gained, fot· the sake of attracting ants and wasps as defenders of the plant • Bedeutung der N l'ktarien,' 1833, pp. 25, 28. Belt nlso refers(' N!caragna,' p. 224) to a sirnilar excretion by many epiphytal orchids and passion-flowers. 1\Ir. Rodgers has seen much nectar secreted from the bases of the ftower-pedllnc1es of Vanilla. Link says that tLe only example of a hypopetalous nectary known to him is externally at tho base of the flowers of CMronia decus.so.ta: see 'Reports on Botany, Ray Society,' 1846, p. 355. An important memoir bearing on this subject has lately appeared by Reinke (' Gottingen NaciJrichten,' 1873, p. 825), who shows t!Jat in mariy plants tho tips of the serrations ou the leaves in the bud boar glands which secrete only at a very early age, and which have the same morphological stmcture as true nectar-secreting glands. He further shows tbnt tl1e n et:JrS! ·creting glands on the petioles of P1·unus avium are not developed at a very early age, yet wither away on tho old leave~:;. 'l'hey are homoloO'ous with those on the serrations of the blades of tho same leaves, as shown by their structure and bv transitionforms; for the lowest senations on the blades of mo,:,t of the leave~ secrete nectar instead. of resin (harz). * I published a brief notice of this case in the 'Gard. Chronicle.' 1855, July 21, p. 487, and afterwards made further observations. Besid s the hive-bC'e, another species of bee, a moth, ants, and two kinds of flies sucked the drops of fluid on the stipules. The larger drops tasted sweet. The hive-bees nev r even looked at the flowers which were opun at the same time; whilst two species of humule-bees neglocte(l the stipulos and visited 011ly the fiowors. 2 D 2 |