THE GRAPE PHYLLOXERA IN CALIFORNIA. 81 
Under average conditions the proportion on the larger roots was 
between 5 and 10 per cent. Regarding the American vines of non- 
resistant type, a considerable diversity in the production of nymphs 
has been observed. On some, like Moore's Early, this production 
may be proportionately very large, while on others, like Isabella and 
Catawba, it may be smaller than on vinifera?, as occurred in the ex- 
periments in caged and potted vines. Vines like Agawam, Lenoir, 
and Delaware, vinifera crosses, bore about the same proportion of 
nymphs as the vinifera?, but among the labrusca types (Isabella, 
Moore's Early, Concord, Champion) there was considerable variation. 
On resistant vines, the nymphs are developed on the nodosities, but 
on vinifera? and American vines of nonresistant type they are also 
produced on other portions of the root system. On phylloxerated 
vinifera?, the most abundant production of nymphs occurs on the 
fleshy and fibrous surface rootlets frequently observable in the 
vineyard. These rootlets are sent out in May and June, and often 
become grossly infested with phylloxera? in June and July. Toward 
the end of July, they decay or dry out, and after that nymphs are 
produced only on the larger roots and on nodosities deeper in the soil. 
On the larger roots relatively few nymphs are produced before August 
or after September. 
Among vinifera? the more vigorous vines produce the greater pro- 
portionate numbers of winged forms. Badly stunted vines showing 
several years of phylloxeration produce comparatively few, while 
the recently attacked vines around the periphery of " spots " produce 
large quantities. Vinifera? vines in pots produce great numbers the 
first year of infestation, but if the soil is unchanged in the second 
and third years, as the vines become weakened, they produce fewer 
winged forms. 
As far as has been observed, all varieties of vinifera? produce the 
same proportion of migrants. 
It has been observed frequently that a humid environment stimu- 
lates the production of migrants and a dry one precludes it. This has 
been especially noticeable in the cases of young vines in pots and of 
the severed roots kept under cellar conditions. The late appear- 
ance of the migrants in the experimental vineyard in 1913 as com- 
pared with those of 1911 and 1915 was perhaps due to lack of moisture 
in the soil in summer. The spring of 1913 was exceptionally dry, 
and the ground became very dry by June, whereas in the two years 
following, moisture was conserved in the top soil until July. The 
total migrant development of 1913, however, although at first re- 
tarded, was finally just about as large as those of the succeeding 
years. To hold the severed pieces of roots, glass jars and dishes were 
used in the cellar, and it was found that in the summer and fall 
1900°— 21 6 
