126 Kelly, On the Function of Acacia Leaf Glands. \yq\^'^xx 



were rarer. That they emerge from their houses is evident, 

 for the outside white coating has been eaten off the gland, and 

 some of the adjacent leaflets. It is at the bitten spots in both 

 instances that the purple colouring and hardening, amounting to 

 lignification, sets in. When this occurs the insect strikes camp. 

 I do not imagine for a moment that the acacias grow these 

 houses for the purpose of accommodating the insects, nor that 

 charity is one of the attributes of plants. They, like every 

 other form of life, are essentially selfish. Nor do they secrete 

 delicacies for these insects ; for whatever they secrete or exude 

 they do for economic purposes and not benevolence. The 

 large hollow thorns of Acacia cornigera, which are said to be 

 inhabited by ants, are regarded as symbiotic structures, and 

 it is generally accepted that mutual services are rendered by 

 insects and plants. Some extra-Australian species are said to 

 secrete sugary substance in their hollow thorns, and it is con- 

 ceivable that the glands in our species are corresponding 

 features aborted or modified. That insects only make use of 

 them, taking them as they find them, is the more acceptable 

 conclusion, until it can be shown that the organ is the common 

 meeting-ground of the symbionts — that each one, if not 

 essential, is at least beneficial to the other. Of this there is 

 very little evidence. Most of the evidence is the other way. In 

 the first place, it seems that only some of the glands are 

 inhabited ; some are almost solid. The insects are only in 

 occupation at certain seasons. Many of the glands excrete 

 their contents in the form of a floccous cap, which evidently 

 loosens and falls away. The insects, if, indeed, they remove 

 the secretion at all, certainly feed upon the tissue of the gland 

 and devitalize it. It is possible that the insect is injurious to 

 the plant. It may be accepted as a fact that the plant is 

 useful to the insect, but that there is no community of service. 

 There is no evidence that the insect is a symbiont by virtue 

 of any protection it affords ; that there is any insect that it 

 wards off, or that it could do so ; or that, if it could, that the 

 insect warded off would be any more injurious or less beneficial 

 to the plant. There is no evidence that the insect is a fer- 

 tilizing agent, or that the plant is entomophilous and not 

 anemophilous, or that this insect is capable of fertilizing 

 flowers. It is more in keeping with my observation that it 

 feeds upon the unopened flower-heads, and in this way is an 

 enemy rather than a friend. At the same time it is probable 

 that the insects are of some, if not great, value in keeping these 

 organs clear of matter, with which they would otherwise be 

 clogged. This service is doubtful, but, if real, would be some 

 indication of symbiosis. I come to the conclusion that these 

 organs perform excretory functions, and are capable of per- 



