54 PROTECTION OF PLANTS, 1917-18 



weather have been collected with great satisfaction. These operations are 

 wonderful, I admit, but I see no advantage in exaggerating their significance. 

 When vigorous aphids are taken captive and die shortly afterwards I conclude 

 that were it not for the ants they would have lived and pumped nourishment from 

 the plant for a much longer period, and would have multiplied to the plant's 

 detriment. We sometimes see these captives dragged away even from indoor 

 plants before the autumn, when there is no fear of severe weather; therefore 

 either the instinct of the ant is at fault, or else what it is seeking for above all is 

 to have its purveyor within reach. In any case it relieves the plant. 



3. The aphis injures the plant in two ways: it withdraws the sap, and it 

 secretes an injurious fluid. This fluid "spreads over the leaves and branches, 

 and by holding the dust and corpuscles floating in the air, blocks up the stomata of 

 the epidermis and often causes the plant to perish." {Provancher: Le Verger Cana- 

 dien, p. 41.) The service rendered to the plant in this case is more direct and I 

 think more important than in the two preceeding cases. However, we must not 

 generalize too much, but rather make some necessary distinctions. Some aphids 

 produce but a small quantity of this sweet liquid; the harm to the plant and 

 consequently the services rendered by the ant are then less appreciable. This 

 is the case, for example, with the aphis rudbeckiae Fitch. As the ants themselves 

 are not numerous they cannot do any considerable damage. When the aphids 

 secrete a considerable quantity of this honey dew tAvo alternatives arise. The 

 aphids are either extremely numerous and completely cover the plant, or are 

 thinly scattered over the branches and leaves. If the aphids be very close to- 

 gether the little drops of the secretion, having no part of the plant to cling to, 

 fall to the ground. At once the sersdce rendered by the ants is considerably 

 diminished. In the second case the drops remain on the plant and the ant is more 

 useful. In both hypotheses there is a sort of compensation: when the aphids are 

 less numerous, the plant suffers more from the secretions; on the other hand, the 

 more numerous are the aphids the more numerous also are the ants, provided 

 other favourable conditions be realized. 



To sum up: the ants help the plant in three ways: first, they accidentally 

 bring about the death of the aphids or at least cause a delay in their injurious 

 action; secondly, they keep them in captivity and force them to observe a fast 

 which is sometimes fatal; thirdly, they absorb the secreted matter which might 

 block the stomata. 



II — Harmfulness of Ants. 



I think I have given the ants sufficient credit for the good they do ; a revelation 

 of their misdeeds will not be out of place. 



First, truth obliges me to say that any service rendered to the plant is alto- 

 gether accidental. The facts given above are an implicit proof of this. If the 



