_ PART III. | THE MECHANISMS OF FLOWERS. — 101 
- sunny weather, when the flowers open, the anthers spread apart a 
little, and dehisce on the side turned towards the stigma. Insects 
which make for the honey must thrust their heads into the flower 
between the stamen and stigma, and bring one side of the head in 
contact with one or two anthers, and the other with the stigma. If 
they move round the flower licking up all the honey-drops, as a 
rule the same side of the head remains turned to the stigma 
_ throughout. In subsequent flowers the opposite side of the head 
' may touch the stigma, and lead to cross-fertilisation. It may 
| happen, and perhaps does so happen frequently, that the insect 
_ plunges its head several times into the same flower, and so leads to 
_ self-fertilisation. In rainy weather the flowers only partially 
_ expand, and the anthers of the longer stamens remain in immediate 
contact with the stigma, and accomplish self-fertilisation. 
Fig. 31.—Nasturtium silvestre, R. Br. 
3 Flower from above. In the centre is the stigma concealing the ovary; around it are seen four 
larger (n) and two smaller (n’) honey-drops. a, the longer stamens; a’, shorter ditto, In all the 
| stamens, the pollen-covered face of the anther is visible, turned towards the stigma. 
, Visitors: A. Hymenoptera—(a) Sphegide: (1) Crabro Wesmeeli, v. d. 
1 L., s.; (2) Tiphia minuta, v. d. L., s.; (b) Apide: (3) Apis mellifica, L. 
| 8, «p., ab.; (4) Andrena Schrankella, K. 9, e.p.; (5) Halictus nitidius- 
| culus, K. 9,8. B. Diptera—(a) Empide: (6) Empis livida, L.,s.; (b) Syr- 
| phide: (7) Syritta pipiens, L.,s. and f.p.; (8) Syrphus sp.,s. and f.p.; (9) 
rf Chrysogaster Macquarti, Loew., s.; (10) Eristalis arbustorum, L., s. ; (c) 
_ Bombylide : (11) Anthrax hottentotta, L., s. 
— _—— Nasturtium officinale, R. Br—There is a remarkable difference 
_ between the flowers of this species and those of JV. silvestre. At 
the inner side of the base of each short stamen are two green 
fleshy honey-glands, placed close together. The shorter anthers 
have their dehiscent sides turned towards the stigma, which stands 
high above them; the taller, which stand at first on a level with 
the stigma, but are afterwards overtopped by it, have their 
| dehiscent sides turned towards the shorter stamens, so that the 
head or proboscis of an insect passing down towards the nectary, 
