858 



ECOLOGY 



Hairs and glandular surfaces. — Stiff bristly hairs have been thought to serve 

 as barriers against various crawling animals, especially snails. Glandular hairs 

 doubtless are still more effective, and it is noteworthy that they abound on floral 

 stems more than elsewhere. Perhaps the most undoubted instance of such protec- 

 tion is in Silene, some species of which (as S. antirrhina) develop just at anthesis 

 an extensive glandular surface on the upper stem internodes; insects are caught by 

 these plants so frequently as to have led to the common name of catchflies. 



Extrafloral nectaries. ■ — On many plants there occur extrafloral nectaries {i.e. 

 nectar-secreting organs apart from inflorescences), as in various legumes, and in 

 Ricinus and Passiflora (figs. 1183, 1184). Usually they are most abundant on the 

 upper side of the petioles and on the under side of the leaf blades. Ants frequently 



1184 



Figs. 1183, 11S4. — Extrafloral nectaries on the leaf of a passion flower {Passijlora) : 



1183, a palmately five-lobed leaf with nectaries (m) on the petiole and also on the blade; 



1 184, a single nectary (») with a large drop of nectar {d) ; considerably magnified. 



visit these nectaries for food, and it commonly has been supposed that the organs 

 thus are advantageous to plants, the view being that the insects are satisfied with 

 what they obtain from the extrafloral nectaries and thus keep away from the flowers, 

 where the rifling of the floral nectaries might endanger cross pollination. It has 

 even been held that nectar-feeding ants are combative and keep off leaf-cutting 

 ants and other harmful insects. There is no valid evidence for these fanciful 

 theories, and recent careful experiments in which plants have been deprived of extra- 

 floral nectaries without affecting seed production or other plant activities would 

 seem to make them untenable. Indeed, the greater frequency of the visits of ants 

 to the nectary-bearing individuals has been shown to lead to more flower-rifling than 

 in the plants deprived of nectaries. In some cases, as in Vicia, bees have been ob- 

 served to visit extrafloral nectaries in preference to floral nectaries; in such a case 

 also extrafloral nectaries are a positive disadvantage to plants. Furthermore, in 



