THE GRAPE PHYLLOXERA LST CALIFORNIA. 25 



of the root system. Since the vine derives its plant food through 

 the growing rootlets that thrust their way through the soil, it is 

 obvious that when such rootlets rot as a result of the decay of the 

 nodosities situated on them no more sustenance can be afforded 

 the plant through this medium. If, on the other hand, the rootlets 

 continue to grow notwithstanding the nodosities situated on them, 

 and if the nodosities lignify, the supply of nourishment provided by 

 the rootlets is not cut off, and the nodosities become in effect tuber- 

 osities. This is often the case with resistant vines, and much more 

 rarely with vinifera or nonresistant Americans. In resistants these 

 tuberosities generally lignify and heal, but in the other types of vines 

 they do this only if their environment is quite dry. Nodosities effec- 

 tively destroy the terminal rootlets; but since the insects spread 

 very slowly on resistants, a vine of any vigor has abundant feeders, 

 and thus it follows that resistant vines bearing very few or no tuber- 

 osities, but having many nodosities, do not succumb to phylloxera. 

 Resistant vines never lack the power to produce enough feeding 

 rootlets to sustain them as long as the following conditions, which 

 are normal to these vines, obtain: (1) When the development and 

 spread of the phylloxeras on them are comparatively slow; (2) when 

 a large percentage of insects that have been raised on the nodosi- 

 ties become nymphs and later leave the roots as winged migrants, 

 in an endeavor to reach the surface of the ground or the aerial 

 parts of the vines. Both of these conditions may be affected by the 

 quality of plant food, as will be shown. Instances have been seen 

 in which young resistant vines have been rid of their entire infesta- 

 tion because all of the immature phylloxeras became winged migrants 

 in the autumn, but in the majority of cases of infested resistant 

 vines under observation there remained in late fall a small wingless 

 infestation, and in some instances where the vines had been growing 

 "in small pots with insufficient nourishment infestations of wingless 

 aphids persisted, and the production of winged migrants during the 

 autumn was proportionately small. These wingless infestations, 

 however, were not prolific. It appears that thrifty resistant vines 

 afford poor nourishment for phylloxeras, and they do not respond to 

 phylloxeric irritation by producing swellings. When, however, re- 

 sistant vines become weakened through a poor supply of plant food, 

 the phylloxeras attacking them persist and the vines respond to the 

 phylloxeric irritation and form lesions. 



Although the decay of the nodosities on vinifera vines destroys the 

 feeding rootlets, this in itself is not a potent factor in the destruc- 

 tion of the vines by phylloxera. Except under abnormal conditions, 

 such as the confinement of vines in pots with impoverished soil, no 

 case has ever been observed in which the death of a vine could be 

 attributed solely to the decay of nodosities, whereas instances have 



