THE GRAPE PHYLLOXERA IN CALIFORNIA. 23 
also occur on the trunk of the vine, both above and below the soil 
surface. They are less commonly formed on roots of one year's 
growth than on older wood. On resistant vines tuberous swellings 
are normally quite unusual, but they may be formed on the healing 
growth of the cambium layer about an abrasion. On most American 
vines of nonresistant type, tuberosities are abundantly formed. On 
viniferaX resistant hybrids the more the resistant strain predominates 
the scarcer are the tuberosities. 
Tuberosities are formed at any time between March and October, 
most abundantly during the summer months. They are formed more 
readily on vigorous roots than on those somewhat dried or decayed. 
Hibernants often choose tuberosities upon which to pass the winter, 
besides inducing their growth at points as yet sound and uninfested, 
the mere insertion of the beak being sufficient to stimulate growth. 
Tuberosities vary considerably in their general appearance, even 
on the same vine. Some are minute papillae on the surface of 
the root. Others are large, fleshy, rapidly growing, globular out- 
growths, as much as half an inch in diameter, and this type is found 
chiefly on the smaller roots. Others are enlargements of the girth 
of the root at intervals, a type also confined to small roots. Others 
consist of more or less uniformly rounded swellings of one-sixth 
to one-fourth inch diameter on the root surface, and these are the 
ones most commonly found on larger roots. Such tuberosities by 
their growth generally split the epidermis of the root longitudinally, 
and as the split tends to lengthen at both ends, the tuberosity assumes 
an oval or elongate shape. Later, when the split enlarges, fresh 
tuberosities are formed by aphids on the inner layer of bark exposed 
by the split, and shortly a chain of lesions occurs along the crack. 
These cracks lengthen and often involve a length of more than 6 
inches. On roots growing horizontally or almost parallel to the soil 
surface, the majority of the tuberosities will occur on the lower side, 
the insects apparently settling there because of the greater moisture. 
On vertical or sloping roots tuberosities occur more or less uniformly 
all around. As long as they remain fresh, tuberosities provide an 
excellent quality of food for the aphids. This condition should be 
distinguished from the rapid development observed in the case of 
aphids settled on root callus, which forms at the point of severance 
and is caused by the action of the healing cells of the cambium layer 
becoming greatly enlarged and very fleshy, furnishing excellent 
food for the aphids, through the natural function of the wounded 
root. 
Many factors influence the length of existence of tuberosities. In 
general, it is found that those formed in the autumn will last until 
the rainy season, and commence to decay immediately afterwards. 
