2Sc} BOTANICAL GAZETTE. 



within the body of the victim, there to undergo its further develop- 

 ment and transformations. These plant-lice were also destroyed in 

 considerable numbers by the larvce of the two-sj^otted lady-bird, which 

 also feeds upon the nectar of the plant. Thus it appears that the se- 

 cretion of nectar by extrafloral glands on poplars attracts to the plants 

 many insects, of which at least three kinds — ants, ichneumonids, and 

 lady-birds — are of benefit to them, the first rendering it unsafe for 

 lepidopterous larvae or other herbivorous insects to frequent the 

 plant, and making contact with it undesirable for larger animals, 

 while the other two destroy one of its insect enemies in large numbers. 



After reaching maturity, the leaves of poplars are quite coriaceous 

 and being for this reason less liable to attack than when younger, no 

 longer require the piotection secured by their glands ; hence it hap- 

 pens that these organs are found actively secreting only on young 

 leaves, and only on those produced early in the season, when the 

 foliage, young fruit, and tender branches most require protection ; 

 and, being no longer required, they are not produced by the later 

 formed leaves. A somewhat similar case is afforded by some of the 

 Smilaccff, where Prof. Delpino'^ has shown that the young plants are 

 protected by a bodyguard of ants, maintained by the i)roduction of 

 nectar by foliar glands, while the older plants, being protected against 

 grazing animals, etc., by their thorns, have no glands. 



Some light is thrown on the conditions upon which the develop- 

 ment of these organs depends, by the drooping aspen already men 

 tioned, turgidity and active growth being evidently the immediate 

 requisites ; but tne primary reason for their existence, bringing in the 

 much-vexed questions, of heredity and first causation, is not so easily 

 cleared up. On the whole it appears probable that these organs are 

 protective, as those of Passiflora, Gossypium and other plants are sup- 

 posed to be ; and we have been able to show that this protective 

 function is, at the present time and in our own climate, of some posi- 

 tive value. ^"^ It is not unlikely, however, that in the Cretaceous and 

 Tertiary periods, in the youth of the genus, protection was far more 

 needed than now, and these glands may then have been efficient 

 in maintaining upon the plant a body-guard of pugnacious ants that 

 served to repel other species which, like the leaf cutting ants against 

 which Ptcris, Acacia, etc., are now similarly protected in tropical 

 America, would have been very destructive to the plants if left to 

 themselves. 



It having been suggested that the glands of Popiihis may have 

 been of great use in the earlier geological ages, the question naturally 



'"' Atti R. Universita di Genova, IV, Pt. I, p. 20. 



'8 Altljous:h of considerable protective value to-day, these glands by no 

 means prevent the plant from suffering defolintion at times. I recollect seeing 

 the cottonwoods {P. ai.guldta) almost entirely stripped of their foliage in Alaba- 

 ma in May, 1879, by a chry.somelid l)eetle, Pldf/itxhrii scriptii. Knowing 

 nothing of the leaf-glands at that tinie,I can not say wliether tliey were active- 

 ly secreting; but I do not recollect seeing ants about the trees, and as I was 

 then carefully studying phenomena of this sort, their presence would scarcel_y 

 have gone unnoticed. 



