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 every 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 
necessary 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 
ee buds than on the older and tougher leaves, which are less tempting 
» herbivorous animals, and more able to resist their attacks: and 
what may be the truth regarding the presence of these glands 
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 leaflets, 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 PROTECTIVE 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 surrounding 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 Droscorea aculeata, the cultivated prickly yam in Guam, 
called ‘* nika,” which it resembles i in the form of its broad he eart-shaped 
iG Rapporti tra insetti e tra nettarii estranuziali, | p. 638, 1875, 
® Naturalist in Nicaragua, p. 218, 1874. 
eSee Cook, An Enemy of the Cotton Boll Weevil, U. 8. Dept. Agr., Rept. No. 78; 
also his Report on the habits of the kelep, or Guatemalan cotton-boll weevil ant, 
U.S. Dept. Agr., Bur. Ent., Bull. No. 49, 1904. 
