J. Davidson 137 



of conclusions being drawn from them. It is intended to repeat these 

 experiments, making the necessary alterations to avoid error. 



During the summer of 1911 a young apple tree infected with Schizo- 

 neura lanigera Hausm. was kept under observation in a greenhouse. 

 The aphids reproduced in great numbers, the leaves began to wilt, and 

 the tree looked very sickly. In the later stages of the attack, winged 

 viviparous females were produced in vast numbers, and most of them 

 left the tree and collected on the woodwork of the greenhouse. Later, 

 all the leaves on the plant died, and although there were several " living " 

 branches the aphids practically all died off. 



The winged forms of Schizoneura lanigera are not common in England, 

 and it would seem that the heavy infestation produced some change 

 in the constitution of the cell sap, which induced the production of 

 winged migrants. When a plant becomes so heavily infested that the 

 leaves begin to wilt and the green parts are smothered with aphids, 

 the normal green surface of the plant which carries on the processes 

 of photosynthesis is considerably reduced in area and as a result the 

 constitution of the cell sap is probably changed. 



It is a well-known observation that aphids prefer the young growing 

 shoots of the plants they attack. They readily select the parts of the 

 plant which afford the best supply of sap. 



In early summer, rose-trees frequently become infected with Macro- 

 siphum Tosae, the aphids collecting on the young stems and terminal 

 shoots, where they reproduce in great numbers. Later on in the season, 

 the trees are often quite free from these aphids. 



During the summer of 1913, one rose-tree in Acton Lodge garden 

 was kept covered with a muslin cage. The tree was covered early 

 in the summer and infected with Macrosiphum rosae. The colonies 

 increased in great numbers on the young terminal shoots, and along 

 the petioles of the flower buds, and soon the plant became very heavily 

 infested, vast numbers of winged forms being produced. Many of the 

 leaves died and the young terminal shoots looked very sickly. Later 

 on practically all the aphids died off. The tree then began to make 

 new growth later in the summer. These young shoots soon became 

 infected and the aphids reproduced in great numbers on them. 



It was noted in the case of the rose-tree experiments mentioned 

 above, that at first the aphids collected on the young terminal shoots, 

 or along the petioles of the flowers, just beneath the buds. When 

 the flower buds were cut off, the aphids very soon left the petioles 

 and went to another part of the plant. Gradually as the numbers 



