External Factors that Influence Growth 273 
the leaves of thenormal plant. Thesame result was obtained with 
the witches broom (Hexenbesen) of the birch (caused by Phy- 
toptus betuli). Similarly for the Bedegar of the rose caused by 
Rhoditis rose. Again, the galls produced on the grass, Poa 
nemorales, by Cecidomyia po, 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 viminalis produccd on Salix purpurea 
fall to the ground in the autumn and may remain alive 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 caprez 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 is 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 thg 
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 froma 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 
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