Nectar absondernde Driisen. 



573 



performing some comparatively simple function. The question at issue may 

 perhaps be stated as follows: In the cases where the nectar-glands are 



now well developed has there been a special course of structural development in 

 close relation with the need of the plant for protection ? Has there been a course 

 of evolution such as we may believe has taken place in the formation of the 

 food-bodies in Acacia splicer ocephala and Cecropia peltata, or should we not 

 rather believe that the sweet secretion has been developed in connection with 

 some unknown process of nutrition ; according to this view, a well developed 

 system of glands may continue merely performing some obscure excretory function, 

 and consequently, although the presence of nectar-glands has undoubtedly been 

 of the utmost importance in determining the survival of certain species, yet it is 

 hardly fair to assume that all nectar glands were originally protective in function. 

 For many plants secrete large quantities of sweet fluid, which serves no such 

 purpose. This argument is given by my father in his "Effects of Cross and Self- 

 Fertilisation" (p. 402). In addition to the facts there given in support of this view 

 a curious case described by Prof. H. Hoffmann may be mentioned ("Ueber Honig- 

 thau," 1876). He states that numerous large drops of sweetish fluid appeared on 

 the under-surface of the young leaves of a camellia. He also alludes to a similar 

 abnormal production of honey-dew on an ivy plant. 



In the case of introduced plants, we see how an already existing quality 

 may, without any special course of development, become of vital importance to 

 its possessor. Thus, Mr. Belt shows ("Naturalist in Nicaragua," p. 74) that the 

 lime, Citrus limonum, is able to exist in a wild state, because its leaves are, from 

 some unknown reason, distasteful to the leaf-cutting ants; whereas the orange, 

 C, aurantium, and the citron, C. medica, can only survive with the help of man. 

 Fritz Muller concludes his letter with some curious facts on kindred subjects : - 

 "The extreme variability of the nectar-glands on the leaves of many plants, 

 is a somewhat remarkable fact. Thus our Citharexylon has normally two large 

 glands at the base of the leaves, but sometimes there is only one, and sometimes 

 none at all ; besides these there are small glands scattered over the surface of the 1 

 leaf, the number of which varies from twenty to none. Similar variations occur 

 in the nectar-glands of Alchornea erythrospermum, and of a Xanthoxylon. It 

 seems to me probable that in all the cases at present known, these glands serve 

 to attract protecting ants ; and I here agree with Delpino, although I do not hold 

 with him that caterpillars are the chief enemies which are guarded against by 

 Pheidole and Crematogaster; but I think with Belt that these latter ants protect 

 the plant against the leaf-cutting species. Indeed it is precisely those plants 

 which are free from the attacks of ants that seem to be especially well fitted for 

 caterpillars. Thus the larvse of Gynaeria live on Cecropia peltata, those of Epi- 

 c.alia numilia on Alchornea erythrospermum. On the Cayien (?) whose leaxcs 

 arc furnished with nectar-glands, and are visited by protecting ants, the cater- 

 pillars of man)' species of Callidryas are found. Finally, as far as I know, all the 

 larvse of the genus Heliconius feed on Passiflora. Moreover, the same relation 

 holds in the case of plants protected in other ways, for instance, by stinging hairs 

 or by poisonous sap. How numerous are the larvae found on the European 

 stinging-nettle. In this country we find the caterpillars of 'Ageronicn' on the 



