48 THE CROWN-GALL AND HAIRY-ROOT OF THE APPLE TREE. 
in a short time, but, except as mentioned, the callus was usually con- 
fined to the wounds and did not project beyond. 
(5) Under very wet conditions, as in water, the formation of callus 
was almost entirely inhibited in wounds, but lenticellular proliferations 
were abundant from other parts of the surface of shoots and roots. 
(6) The two regions where the formation of callus in root grafts 
was most abundant, the lower point of the scion piece and the lower 
end of the root piece coincide exactly with the position of more than 
90 per cent of the galls occurring on root-grafted trees, indicating a 
close relation between the formation of callus and the development 
of the disease. 
Fifty root grafts with large outgrowths of callus (Pl. VIII, fig. 2) 
were selected from those in the experiment and the same number 
with no excessive development of callus were selected. These were 
planted in separate benches in a greenhouse in ordinary soil, not ster- 
ilized. They all grew. Of the set with excessive callus 32.2 per cent 
developed galls of the hard form. All of the other set grew into 
healthy trees. This indicates that excessive callus formation either 
is Incipient crown-gall or may develop into it. Storage conditions 
should be such as to hinder the excessive development of callus. 
Moist sand is the best medium of those tested for packing root grafts. 
Studies on root formation as related to havry-root.—In connection with 
the study of the formation of callus and under the same conditions, 
an experiment was conducted for the study of root production on 
healthy seedlings as compared with that on those diseased with hairy- 
root and the woolly aphis. Seedlings of these three types were pre- 
pared as in the former experiment and placed under the same con- 
conditions of moisture. 
It was found that as a rule no roots were produced from cuttings 
or scions from the limbs of healthy apple trees under the conditions 
of the experiment. Such was not the case with cuttings taken from 
the tops of a number of apple seedlings badly diseased with the simple 
form of hairy-root. Roots were produced freely from all of these 
under moist conditions. Where healthy apple seedlings in which the 
stem and root tip had not been cut off were kept in a cool condition 
for two months and in a warm condition for one month, a few scat- 
tered new side roots developed on most of the roots, but only shoots 
or leaves developed from the stems. In the case of the seedlings dis- 
eased with hairy-root which were placed under similar conditions, 
abundant root formation, usually in clusters or tufts, took place from 
tissues in or adjacent to. the diseased parts, not only from the roots 
but often from the stems, and at the same time shoots were thrown 
out from the root tips below diseased areas. This is a reversal of the 
natural order of root and shoot formation in apple seedlings. 
186 
