68 USEFUL PLANTS OF GUAM. 



On this account Delpino'' argues that these glands ought not to be 

 regarded as excretory, since if they were so, they would be more 

 constant and would occur in eveiy species. Their variability is 

 especially noticeable in the genus Cassia, where the tiny cup-shaped 

 nectaries may be found on the petioles of some species and the rachis 

 of others, but are absent from both in others. If they performed some 

 necssary function it is hard to believe that they would not occur in 

 all the species. One thing is certain, they are more highly developed 

 and more active in the young and tender leaves and about opening 

 leaf buds than on the older and tougher leaves, which are less tempting 

 to herbivorous animals, and more able to resist their attacks; and 

 whatever may be the truth regarding the presence of these glands in 

 general, Belt has shown conclusively* that the bull's-horn acacia of 

 Central America {Acacia sphaerocephala) not only attracts stinging ants 

 by its nectaries, but offers them as an additional attraction dainty food 

 rich in oil and protoplasm in the form of small bodies at the end of the 

 divisions of the compound leaiflets, which the ants gather when ripe 

 and carry to their homes in the stout hollow thorns of the plant itself. 

 The fruit-like bodies do not ripen all at once, but successively, so that 

 the ants are kept about the young leaf for some time after it unfolds, 

 and Belt arrived at the conclusion that the ants are really kept by the 

 acacia as a standing army, to protect its leaves from the attacks of 

 herbivorous mammals and insects. In the same way there is a succes- 

 sion of active nectaries about the tender young leaf buds and flower 

 clusters of Ricinus, which are constantly visited by wasps and ants; 

 and the important part played by the nectar glands in the petioles of 

 the cotton leaf (PI. X) as an attraction to ants which serve to protect 

 the plant from the boll weevil and other injurious insects has recently 

 awakened great interest and has been turned to economic account." 



PLANTS WITH PKOTEOTIVE DEVICES. 



Interesting examples of self-protection are offered by several plants 

 growing in Guam, the most striking of which is that of the spiny yam, 

 Dioscorea spinosa. This plant grows spontaneously on the island and 

 in places forms impenetrable thickets. It takes its name not from the 

 small prickles on the stem but from a mass of spines stirrounding the 

 base of the stem and serving as a protection to the starchy tubers 

 below from hogs and other enemies. This species has often been con- 

 fused with Dioscorea aculeata, the cultivated prickly yam in Guam, 

 called " nika," which it resembles in the form of its broad heart-shaped 



« Rapporti tra insetti e tra nettarii estranuziali, p. 63, 1875. 



* Naturalist in Nicaragua, p. 218, 1874. 



cSee Cook, An Enemy of the Cotton Boll Weevil, U. S. Dept. Agr., Rept. No. 78; 

 also his Report on the habits of the kelep, or Guatemalan cotton-boll weevil ant, 

 U. 8. Dept. Agr., Bur. Ent., Bull. No. 49, 1904. 



