284 TRANSACTIONS OF ROYAL SCOTTISH ARBOKICULTUKAL SOCIETY. 



the twisted or curved shape lasts for many years after the 

 injury. 



Galls. 

 A gall is generally known as an abnormal growth of some plant 

 tissues produced by animal agency acting from within. The 

 exciting cause in the case of typical galls appears to be a minute 

 quantity of some irritating fluid or virus secreted by the female 

 insect, and deposited in the cortical or foliaceous parts of the 

 plants. The virus causes the rapid enlargement and subdivision 

 of the cells affected by it, so as to form the tissues of the galls. 

 Larval irritation plays an important part in the formation of 

 many galls. 



Chermes abietis — Spruce Gall Aphis. 



This very minute insect, of a yellowish green to a light brown 

 colour, with whitish down and white wings, punctures the young 

 buds for the purpose of depositing its ova. They have passed the 

 winter in crevices and under bark scales, and early in April attach 

 themselves each to the base of a young leaf, which reacts by a 

 small swelling. There they lay a number of eggs. By means of 

 the puncture thus made, and the absorption of sap by the young 

 aphides, a cone-like swelling of the compressed needles takes place, 

 green at first but afterwards ruddy brown. Each gall contains a 

 series of cavities or chambers. In these cavities the larvfe live in 

 numbers, either entering the chambers during the growth of the 

 gall or being enclosed by the swelling of the surrounding needles. 

 Fortunately the side shoots are much more frequently attacked 

 than leading shoots, for the shoot when punctured assumes curious 

 shapes and bends, and if badly attacked the young plants fall into 

 a sickly condition. Such galls are now very frequent. They 

 should be cut off and burned ; by doing so twenty or thirty larva? 

 in each are killed. This would also not only save spruce, but 

 larch, as the spruce aphis is understood to be one stage in the 

 life-history of the larch aphis. 



Chermes laricis — Larch Aphis. 

 This small dark brown insect, easily recognisable from its white 

 woolly pad, damages the needles of the larch, from April to August 

 by sucking the sap, and when the insects attack the leaves they 

 render them incapable of perforniing theii- functions, and they 

 may leave wounds l)y which the spores of injurious fungi may 

 gain entrance. 



