NECTAR OF FLOWERS 95 
are only perceived by these special visitors, or are at any rate peculiarly sympathetic 
to them. For example— 
Andrena florea /’. is found exclusively in Bryonia dioica Jacg. (vide supra); 
Andrena Hattorfiana /. and Cetii exclusively on Knautia arvensis Cozlt.; 
Andrena Nasuta G7r. exclusively on Anchusa officinalis Z. ; 
Bombus Gerstaeckeri Jor. exclusively on Aconitum Lycoctonum Z. ; 
Cilissa melanura /Vy/. almost exclusively on Lythrum Salicaria Z. ; 
Macropis labiata Pz. almost exclusively on Lysimachia vulgaris Z. ; 
Osmia adunca Za/r.and Caementaria Gers/. almost exclusively on Echium. 
On the other hand, bees of the genus Prosopis, which themselves possess 
a strong odour, prefer to seek out flowers that have a powerful smell, due to their 
containing ethereal oils—e.g. Ruta, Anethum, Reseda, Lepidium, Achillea, Matricaria. 
Many flowers are almost entirely scentless during the day, and exhale a very 
strong odour in the evening and at night. These are, without exception, moth 
flowers (see p. 67). 
Flowers offer Pollen, and usually also Nectar or enclosed Sap, as food to the 
insects that have been enticed by colour or odour ; and in return for this the visitors, 
as a rule, effect transference of pollen, cross-pollination being thus brought about in 
many cases. Sometimes flowers also afford insects Shelter, which they can 
leave again at pleasure, or else they are compelled to make an unwilling stay of 
considerable duration. 
Pollen alone, as a reward for work done, is offered by relatively few flowers 
to the insects that visit them and thereby effect pollination. These Pollen Flowers 
will be considered more fully later on (cf. p. 105). 
Secretion of Nectar usually takes place deep down in the flower by means 
1 2 
Fic. 12. Wectaries of some Ranunculaceae. (Enlarged. From Nature.) 
1. Ranunculus sceleratus, L. 4. Aquilegia vulgaris, L. 
2. Trollius europaeus, L. 5. Aconitum Napellus, L. 
3. Helleborus niger, L. 6. Nigella arvensis, L. 
n, Nectary. ~, limb. s, stalk. d, cover. h, protuberance. 
of special glands (nectaries). These are sheltered in the most varied ways: between 
a fully-exposed position, e.g. in most of the Umbelliferae, and concealment in long 
corolla-tubes (Lonicera Caprifolium), or in long spurs (Corydalis cava), there are 
numerous gradations, so that Herm. Miiller has established eight classes of flowers, 
according to the position of the nectar, and the insect visits which are determined by 
