24 BULLETIN 903, U. S. DEPARTMENT OF AGRICULTURE. 
Their decay is expedited by a heavy rainfall and a high-water table. 
Those formed during the spring and summer in a moist environment 
rarely persist fresh beyond two months, and most of them decay 
about one month after they arise. It has been repeatedly observed 
how quickly a fresh tuberosity decays when it is placed against wet 
sand, and if a stream of water finds its way down a root the tuberosi- 
ties thereon start to decay immediately. On the other hand, they 
are more capable of withstanding dry soil conditions than are the 
nodosities, and under conditions approaching drought, which some- 
times occur in late summer and autumn, may last for a considerable 
time and even lignify. the dry environment having caused the insects 
settled on them to seek more favorable conditions of moisture and 
at the same time having kept in check decomposition. Tuberosities 
withstand a considerably greater range in temperature than do 
nodosities, and they are not affected by sudden changes in tempera- 
ture hi the same manner as are the nodosities. 
Tuberosities grow larger and more rapidly in proportion to the 
soundness of the roots. On roots previously uninfested the growth 
of the swellings is rapid and vigorous, and a root, after it has been 
heavily phylloxerated for several months, becomes so greatly ex- 
hausted that it can not respond to the punctures of the aphids by 
developing new swellings, and the phylloxera? that are not gradually 
driven away to seek more nutritious food develop on the root without 
causing swellings. The decay of the tuberosities begins at the place 
first punctured by the aphids. generally at about the center of the 
swellings. The tuberosity forms around the insect, and decay is 
first evident as a small, blackened spot, sometimes exuding a liquid. 
The rapidity of decay of tuberosities is in proportion to the increasing 
moisture content of their environment, and in an unusually dry 
environment they frequently will lignify without causing the tissues 
to rot. Under moist conditions the inflated cells rapidly break down 
and decay usually spreads, and fungi and molds enter the tissues, es- 
pecially in the case of large bulbous swellings. Decay finally drives 
off the aphids. but through their stimulating action they are often 
able to retain the freshness of a tuberosity for some time after it 
has been surrounded by decayed tissues, and occasionally a fresh, 
vigorous specimen is found on a root otherwise quite decayed. The 
nutritious quality of these tuberous lesions provides for the produc- 
tion of nymphs in great numbers. 
HOW ROOT LESIONS AFFECT THE HEALTH OF VINES. 
It has been shown in the foregoing pages that the nodosities are 
those phylloxera lesions formed at the apex of growing fibrous root- 
lets, whereas the tuberosities are lesions formed on all other parts 
