w 
partim.| § THE MECHANISMS OF FLOWERS. 249 
by a white, fleshy disk on the upper surface of the ovary. Insect- 
visitors, collecting or feeding on pollen or thrusting their heads 
_ down between the stamens towards the honey, can thus lead to 
_ cross-fertilisation only in young flowers, but in old ones cross- 
fertilisation and self-fertilisation are alike possible. In absence of 
insects, self-fertilisation may easily take place, for the stigmas 
stand partly in the line of fall of the pollen. But the honey, only 
hidden by the stamens, is accessible to so many insects, and the 
white colour and strong scent are such powerful attractions, that in 
fine weather insect-visits can seldom fail. 
Tribe Ribesien. 
_ 158. Rises ALpinuM, L.—This species presents its honey in a 
shallower depression than any of our other species of Ribes, so 
Fic. 81.—Ribes alpinum, L. 
1.—Female flower, from above, 
2.—Ditto, in section. 
3.—Male flower, in section. 
a’, rudimentary anthers ; st’, rudimentary stigma; n, nectary. 
' that it is easily accessible to very various insects, and is con- 
| sequently visited by the most manifold forms. Although I only 
had an opportunity of watching a few plants in a hedge, I saw 
upon their flowers the following : 
\q = A. Hymenoptera—A pide : (1) Andrena albicans, K. 9 ¢, s. and f.p., very 
~ ab.; (2) A. Gwynana, K. ¢,s.; (8) A. nana, K. ¢,s. ; (4) Halictus nitidus, 
| Schenck, ?, s.; (5) H. nitidiusculus, K. ?,¢.p.; (6) Sphecodes gibbus, L. 
' 9, s. B. Diptera—(a) Muscide: (7) Scatophaga ‘stercoraria, L.; (8) S. 
merdaria, F.; (b) Syrphide: (9) Syritta pipiens, L., all three ab., s. 
: Upon this abundance of insect-visits, which insures cross- 
fertilisation, depends the dicecism, evolved from a previous 
_ hermaphroditism and still showing traces thereof, which would 
\ of necessity be fatal in default of insect-visits. The male flowers 
are of scarcely larger size, but their greenish-yellow colour is 
) more conspicuous than the green female flowers. The calyx 
| almost alone plays the part of signal to the insects. | 
