34 ON PLANTS AND INSECTS. [LECT. 



and the period at which the visits of insects are of 

 importance to the plant. Sprengel was the first to 

 point out the real office of honey, but his views were 

 far from meeting with general assent, and, even as 

 lately as 1833, were altogether rejected by Kurr, who 

 came to the conclusion that the secretion of honey is 

 the result of developmental energy, which afterwards 

 concentrates itself on the ovary. 



No doubt, however, seems any longer to exist that 

 Sprengel's view is right ; and that the true function of 

 honey is to attract insects, and thus to secure cross- 

 fertilization. Thus, most of the Rosacese are fertilized 

 by insects, and possess nectaries ; but, as Delpino has 

 pointed out, the genus Poterium is anemophilous, or 

 wind-fertilized, and possesses no honey. So also the 

 Maples are almost all fertilized by insects, and produce 

 honey ; but Acer negundo is anemophilous, and honey- 

 less. Again, among the Polygonacese, some species are 

 insect-fertilized and melliferous, while, on the other hand, 

 certain genera, Rumex and Oxyria, have no honey, and 

 are fertilized by the wind. At first sight it might 

 appear an objection to this view, and one reason 

 perhaps why the earlier botanists missed the true ex- 

 planation may have been the fact, that some plants 

 secrete honey on other parts than the flowers. Belt 

 and Delpino have, I think, suggested the true function 

 of these extra-floral nectaries. 1 The former of these 

 excellent observers describes a South American species 

 of Acacia : this tree, if unprotected, is apt to be stripped 



1 I by no means, however, wish to suggest that we as yet fully 

 understand the facts. For instance, the use of the nectary at the 

 base of the leaf of the fern is still quite unexplained. 



