CROWN-ROT OF FRUIT TREES: HISTOLOGICAL STUDIES 493 
discoloration in the wood. It should be noted also that at least one 
radial cleft occurred in the bark of the tree represented in Fig. 49: 
the fusion is still incomplete. However, in the tree, a section of 
which is shown in Fig. 50, a portion of the affected bark died and the 
entire wood cylinder, up to the line represented by oc in Fig. 18 
(the outer boundary of the wood at the time of injury), has completely 
decayed. In a few points, just as in Figs. 3, 39 and 42, some of the 
wood produced afterward also died and decayed. This shows that 
decay depends as much upon the death of the wood as upon the 
presence of wood-rotting fungi. Bark' and wood having groups or 
streaks of dead tissue naturally contain relatively large quantities of 
air, and sapwood dying from such bark injuries is full of both air 
and stored food. The high air content of such wood led Miinch^ to 
conclude that the presence of the excessive air is the factor that permits 
wood-rotting fungi to vegetate actively in such tree trunks. Based on 
these conclusions of Miinch, Appel^ has been led far afield in his 
discussion of the factors governing the activity of wood-rotting fungi. 
The fact that such wood is killed while it is young and full of stored 
food makes it evident that it differs materially from ordinary heart- 
wood that has become depleted of most of its stored food (including 
the layers of hemicellulose usually present on the inside of its cell- 
walls) before it became lifeless. It seems more likely that wood- 
rotting fungi thrive uncommonly well in such wood because it contains 
large quantities of stored food and masses of more or less disorganized 
and therefore non-resistant protoplasm, rather than because of the 
great abundance of air present. 
The small apple tree shown in Fig. 46 and the large ones of Figs. 
47 and 51 are examples in which the most .severely affected bark died. 
In those shown in Figs. 26 and 51a complete girdle is involved, while 
in that of Fig. 47 only about three fourths of the bark succumbed. 
Comparison of Effects on Large and Small Trees. — ^The initial 
injuries, from which crown-rot and some other bark diseases arise, 
are the same on large and small trees; the differences usually 
noticed afterward result from subsequent changes owing to differ- 
ences in the thickness of the bark and in the diameter of the 
5 Miinch, E., Untersuchungen iiber Immunitat und Krankheitsempfanglichkeit 
der Holzpflanzen, Naturw. Zeit. Forst. Landw. 7: 54; 87; 129. 1909. 
^ Appel, O., The Relations between Scientific Botany and Phytopathology, 
Ann. Mo. Bot. Gard. 2: 275. 1915. 
