THE GEAPE PHYLLOXERA IX 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 papillse on the surface of 

 the root. Others are large, flesh y, 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 decav immediatelv afterwards. 



