THE GEAPE 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 larvse. The phylloxerse 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 phylloxerse 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 phylloxerse 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 phylloxerse. 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 larvse 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. 



