110 
THE AMERICAN BOTANIST. 
when the sun is shining and these insects must serve as a 
protection for them. It is interesting to note that these 
glands may occur in one species and be absent from an- 
other closely allied to it, of the same genus. Indeed, there 
are species in which the glands are present on some leaves 
and absent from others and of their variability we have 
already spoken in connection with Ricinus and Urena. 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 on the rachis of others, 
but are absent from both in others. If they perform some 
necessary function it is hard to believe that they would 
not occur in all 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 herbi- 
vorous 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 buirs-horn acacia {Acacia sphserocephala) of Central 
America 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 leaves for some time 
after they are unfolded and Belt arrives at the conclusion 
that ants are really kept by the acacia as a standing 
army to protect its leaves from herbivorous mammals 
and insects. In the same way there is a succession of 
active nectaries about the tender young leaf-buds and 
flower cluster of Ricinus which are constantly visited by 
wasps and ants ; and the important part played by the 
