External Factors that Influence Growth 273 



the leaves of the normal plant. The same result was obtained with 

 the witches broom (Hexenbesen) of the birch (caused by Phy- 

 toptus betuH). Similarly for the Bedegar of the rose caused by 

 Rhoditis rosse. Again, the galls produced on the grass, Poa 

 nemorales, by Cecidomyia poae, send out rootlike processes, and 

 if these are covered with earth, they produce true roots that re- 

 semble histologically those of the grass in every respect. Thus 

 the gall itself stimulates a part of the plant to produce roots 

 that never does so normally, and these roots are similar to the 

 normal roots of the same plant, although arising from the gall. 

 The galls of Nematus viminahs produced on Salix purpurea 

 fall to the ground in the autumn and may remain ahve through 

 the winter. In the spring they may increase in size, develop 

 more chlorophyll, and produce lenticels over the surface. 

 Beyerinck succeeded, by keeping these galls on moist sand, in 

 causing them to produce roots from the inner surface, and these 

 may even protrude through the opening in the gall. The roots 

 are like the normal roots of the plant that produced the gall. 



The nature of the substance injected along with the egg into 

 the leaf by Nematus capre^e is unknown, except in so far that it 

 is an albuminous matter secreted by glands connected with the 

 ovipositor. Its amount is exceedingly small, both the egg and 

 the surrounding secretion measuring not more than 0.06 milli- 

 meter, and more than half of this mass i s taken up by the egg. 

 The gall produced is about 10 millimeters in diameter. The 

 disproportion in size is so great that Beyerinck suggests that the 

 substance injected contains an enzyme that acts on the cells of 

 the plant and excites them to the growth that leads to the 

 formation of the gall. 



Some species of Aphids and of Phylloxerans also produce galls, 

 many of them of remarkable size and beauty. The stem-mother 

 after emerging from a winter egg crawls out on to the young leaves 

 and affixes herself at one spot on the under surface, and begins 

 to suck the juice of the plant. Her presence, or more probably 

 some secretion that flows from her proboscis into the wound, ex- 

 cites growth on one or on both sides of the leaf. A hollow gall 



