THE GRAPE PHYLLOXERA IN CALIFORNIA. 59 
rootlets around the inside of the pots, showed that such a penetration 
had occurred. It was obvious in these instances, few though they were, 
that the heavy infestations could not have resulted from inoculations 
on rootlets from wanderers, because only a few nodosities occurred 
on the rootlets, showing a slight wanderer infestation, and not 
enough time had elapsed in the interim for the infestation present 
at the date of examination to be produced by so small a company of 
wandering larva?. The phylloxera? had no difficulty in finding their 
way through the upper cylinder to the root system of the upper pots 
through layers of cotton and sand each about half an inch thick. 
On the roots of the upper pot of cage V, Burger, there were, on 
November 26, 1913, upward of 1,600 hibernants disposed in large 
clusters on the main root. Since June of that year, the visible por- 
tions of four roots below had been well infested. Every one of those 
1,600 hibernants was the progeny of phylloxera? hatched on the visi- 
ble part of the roots and which had penetrated the upper cylinders. 
It is obvious that a great many individuals penetrated the cylinders, 
as the scarcity of lesions showed that the greater part of the in- 
festation was comparatively recent. Apparently a natural law 
against overcrowding comes into play, and migration was encouraged 
by the fact that the tuberosities on three of the four roots had become 
rotted and threatened to decay all the visible portions of those roots. 
As, on this vine, no infestation other than a few nodosities occurred 
below the cylinders in the lower pots, it would appear that the heavy 
migration had been entirely in an upward direction. As far as could 
be determined, there appeared no reason why the insects could not 
penetrate the lower cylinders just as easily as the upper ones, so the 
conclusion is that in most cases they did not make the attempt. 
In the instances wherein phylloxera? had undoubtedly penetrated 
the lower cylinders they were found to be close to the cylinder as 
if the packing of sand and cotton had been so loose that no effort 
was needed for the insect to force its way through. The sand 
in the upper cylinders, by reason of the weight of earth pressing 
upon it, always remained well packed and presented a barrier to the 
progress of the phylloxera?. That they were able to surmount this 
barrier is shown by the large numbers present, and indicates that 
the upward migration was a well-defined movement. The possibility 
presented itself that infestations on the roots of the upper pots could 
have originated from wandering larva? that had penetrated the 
soil of the upper pots in the same manner as they had obviously done 
in the lower pots of the cages. The absence of cracks except around 
the periphery of the soil in the pots and of nodositous infestations 
on the rootlets below taken in conjunction with the size of the infesta- 
tions precludes this as the sole source of the inoculations of the upper 
pots. 
