EXTRA-FLORAL NECTARIES 



OF THE many adaptations of 

 flowers to secure that cross- 

 pollination which ordinarily 

 seems to be so advantageous to 

 them, none is of more obvious use than 

 the nectary. If it is desirable for the 

 flower to attract insect visitors, surely 

 nothing could be more attractive than 

 a reservoir or fountain of nectar, from 

 which they can drink. 



Early botanists indeed were consider- 

 ably puzzled by the case. Patrick Blair 

 guessed that the honey absorbed the 

 pollen and thus fertilized the ovary; 

 Pontedera thought it kept the ovary in a 

 moist condition: Linnaeus "gave it up;" 

 "Krlinitz thought he observed that in 

 meadows much frequented by bees the 

 plants were more healthy, but the 

 inference he drew was, that the honey, 

 unless removed, was very injurious, 

 and that the bees were of use in carrying 

 it off." 



But since the cross-pollination of 

 plants, and the part that insects play 

 in it, have been understood, there has 

 been little doubt as to the utility of 

 nectaries in most flowers. 



When, however, we come to nectaries 

 outside the flowers, such as not a few 

 plants possess, we are obliged to pause 

 for consideration. Such nectaries can 

 be seen to good advantage on the leaf- 

 stem of any cherry, on the passion 

 flowers (for example, the "may-pop," 

 Passiflora incarnata, of the United 

 States), on the cowpea and various 

 other legumes, on the castor bean 

 {Ricinus communis) , and on many other 

 less common plants. The accompany- 

 ing photographs show the extra-floral 

 nectaries at the base of cherry leaves, 

 where there are usually two, and 

 sometimes half a dozen, which may be 

 not only on the petiole, but on the mar- 

 gin of the leaf itself. 



The early Darwinian school of natur- 

 alists, who felt it necessary to find a 

 purpose for the existence of every 

 feature of an organism, in order to 

 account for its development through 

 natural selection, spent a good deal of 



ingenuity on the extra-floral nectary. 

 In the numerous cases where this 

 gland is on the flower-stem or close to 

 the flower, it was suggested that its 

 purpose was to act as a sort of "blilid" 

 for ants, which might otherwise enter 

 the flower itself, in their search for 

 honey, and thus self-pollinate it. If 

 they found a nectary at the base of the 

 flower, it was argued that the ants 

 would satisfy their needs from it, and 

 depart without upsetting the flower's 

 plans for cross-pollination. 



The idea that extra-floral nectaries 

 were a special appeal to ants was 

 developed most fully by Belt and 

 Delpino, the former of whom about 

 forty years ago published his description 

 of the remarkable bull-horn acacias of 

 Central America, in the huge spines of 

 which colonies of ants make their 

 homes, feeding on syrup from extra- 

 floral nectaries, and paying for their 

 entertainment by protecting the tree 

 from all other living things. Describing 

 these acacias, W. E. Safford writes: 

 "The bi-pinnate leaves have nectar 

 glands on the rachis and petiole, as in 

 many other acacias and they are still 

 further provided with peculiar processes 

 on the tips of the leaflets, minute, wax- 

 like bodies rich in oil and protoplasm, 

 which Thomas Belt in his Naturalist in 

 Nicaragua (1874) discovered to be 

 used as food by the ants inhabiting the 

 spines, and which in his honor were 

 named Beltian bodies." F. Delpino, 

 in a classical work on the subject 

 {Funzione mirmecofile nel regno vegetale, 

 1886-9) brought together many other 

 illustrations of these supposedly pro- 

 tective nectaries outside the flower 

 proper, enumerating their occurrence in 

 3,030 species of 292 genera, of which 

 563 are in America. The greatest 

 number is in the pea family (Legumin- 

 osae), closely followed by the Euphor- 

 biaceae or spurge family. 



Microscopical examination of these 

 nectaries at various stages shows that 

 their development takes quite different 

 courses in different species. There 



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