386 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1921. 



observations are Acacia nicoyensis, a plant allied to Acacia sphaero- 

 cephala, bearing large, inflated, spine-pointed pods, and Acacia cos- 

 taricensis, with bivalved pods which split open at maturity. Both 

 of these species were described by Dr. Heinrich Schenck. 5 Photo- 

 graphs of them are reproduced in the present paper on plates G 

 and 7. 



CHARLES DARWIN ON BELT'S OBSERVATIONS. 



In discussing the nature of extra floral nectaries Charles Darwin 

 calls attention to the observations of Belt quoted above. After ques- 

 tioning Delpino's assertion that the power of secreting a sweet fluid 

 by any extra-floral organ has been in every case specially gained for 

 the sake of attracting ants and wasps as defenders of the plants 

 against their enemies, and that such organs ought not to be consid- 

 ered simply as excretory, Darwin admits that the nectar does serve 

 to attract insects which defend the plant and that the glands may 

 have been developed to a high degree for this special purpose, as in- 

 dicated by Belt's observations on the bull-horn acacias of Nicaragua. 

 And he also refers to the apical bodies on the leaflets rich in oil and 

 protoplasm, which are an additional attraction to the ants. 6 



FRANCIS DARWIN ON THE NATURE OF THE NECTAR GLANDS AND 



FOOD BODIES. 



Francis Darwin in describing the main extra floral nectary on the 

 petiole of Acacia sphaerocephala, or a species closely allied to it, com- 

 pared its form to that of a flat thorn, " such as those on roses," with 

 the top cut off; a miniature volcanic mountain with an elongated 

 crater like a narrow trough running along the ridge summit of the 

 gland, in which the nectar wells up like lava from the subadjacent 

 secreting tissues, sometimes in such abundance as to overflow and 

 drip to the ground. On plate 2 is shown a leaf of Acacia cornigera 

 L. from a plant growing in a greenhouse of the United States De- 

 partment of Agriculture, Washington, D. C, with a single elongated 

 nectary on the petiole and numerous apical bodies on the leaflets. 

 Figure 3 is a cross section through the petiole and nectar gland. 



The apical appendages on the leaflets, which have been designated 

 " Beltian bodies," measured 2 millimeters in length. Francis Dar- 

 win describes them as shaped like a pear with one side much flat- 

 tened. On examination with a microscope he found them to be com- 

 posed of cells containing a granular protoplasmic body in which oil 

 globules were imbedded. Their structure he compares to that of 



5 See H. Schenck, Die myrmekophilen Acacia-Arten. Engl. Jahrb. 50. suppl. pp. 449 to 

 487. 1914. 



• Darwin, Charles. Cross and Self Fertilization In the Vegetable Kingdom p. 404. 

 1877. 



