Nectar absondernde Drusen 1 ). 



(Brief an Fr. Darwin und Erwiderung.) 



I have briefly described in vol. XV. of the Linnsean Society's Journal, the 

 nectar- glands found at the base of the fronds of the brake fern (Pteris aquilind) 

 which are visited by ants for the sake of their sweet secretion. This case seemed 

 to me to show in a striking manner that extra-floral nectar-glands are not 

 necessarily protective in function, because the fern has, in England at least, ex- 

 tremely few enemies. The following extract of a letter lately received from Fritz 

 Miiller (of St. Catharina, Brazil) is of considerable interest in relation to this sub- 

 ject He states that 



"the honey-glands on our Pteris aquilina serve, without doubt, to protect 

 the ferns from the depredations of the leaf-cutting ants ((Ecodoma), as is the case 

 with Passiflora, Luffa, and many other plants. The glands of the Pteris are 

 eagerly visited by a small black ant, Crematogaster, of which the (Ecodoma 

 seems to stand in great dread. On the other hand, when no protecting ants are 

 present, I have seen (Ecodoma gnawing the young fronds; here, as in other cases, 

 it is only the young leaves that stand in need of protection, the older ones not 

 being attacked by the leaf-cutting ants." 



This fact might, no doubt, be used as an argument by those who believe 

 tlmt all nectar-glands were originally developed as protective organs, and this 

 argument would have great force if it could be shown that Pteris aquilina is a 

 form which has arisen in countries where protection is needed; but even in that 

 case there would remain the difficulty of accounting for the continued functional 

 activity of the glands in districts where no such protection is required. Or 

 it may be said that in past ages the glands on our European Pteris served 

 .is a protection against enemies which have now become extinct. But here we 

 are again met by the difficulty of accounting for the continued activity of 

 the glands. It is characteristic of evolution that great changes occur in the 

 functions of organs, and J think that it will generally be allowed that even 

 the the most beautifully adapted apparatus must have originated in an organ 



i) Nature 1877. Bd. XVI. p. 100 u. 122. 



