NECTAR OF FLOWERS 



95 



are only perceived by these special visitors, or are at any rate peculiarly sympathetic 

 to them. For example — 



Andrena florea F. is found exclusively in Bryonia dioica/ac^. (vide supra); 



Andrena Hattorfiana F. and Cetii exclusively on Knautia arvensis Coult. ; 



Andrena Nasuta Gir. exclusively on Anchusa officinalis L. ; 



Bombus Gerstaeckeri Mor. exclusively on Aconitum Lycoctonum Z. ; 



Cilissa melanura Nyl. almost exclusively on Lythrum Salicaria L. ; 



Macropis labiata Pz. almost exclusively on Lysimachia vulgaris L. ; 



Osmia adunca Latr. and Caementaria Gersi. 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 



Fig. 12. Nectaries of sOTjte RanuncJilaaae. (Enlarged. From Nature.) 



1. Ranunculus sceleratus, L. 4- Aquilegia vulgaris, L 



2. Trollius europaeus, L. 5- Aconitum Napellus, L. 



3. Helleboriis niger, L. ■ 6. Nigella arvensis, L. 

 », Nectary. p, limb. s, stalk. d. cover. /i, 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. Muller has established eight classes of flowers, 

 according to the position of the nectar, and the insect visits which are determined by 



