CROWN-ROT OF FRUIT TREES: HISTOLOGICAL STUDIES 495 
woody plant the development of primary bark is supplanted by the 
formation of secondary bark, apparently because of the inability of 
the primary tissues to continue their adjustment to the increase of 
the stele. This transition stage in trees usually occurs at an age 
characteristic for the species. Afterward the bark continues to in- 
crease in thickness, and to be distended, growing somewhat radially 
for a certain number of years; then a new phellogen is again formed 
inside that portion of the bark that is no longer able to undergo suffi- 
ciently rapid tangential growth, and a fresh portion of lifeless bark 
results which is more or less continuous around the trunk — the results 
in this respect also varying with the species. 
It appears that there is a close relation between the growth cycles 
mentioned above and the periodicity that may usually be noticed in 
regard to bark cycles. The growth and cell-size minima seem usually 
to fall in the season just preceding a new period of bark-roughening, 
while the maxima are usually reached during the second third of the 
time elapsing between the occurrence of two minima. The environ- 
mental variations, and, in case of cultivated trees, the culture and 
tilth given, have a marked influence upon the prominence of this 
periodicity. 
In a smooth-barked stem that portion of the bark outside the oldest 
circle of sclerenchyma (the cortex) often undergoes much, although 
limited, growth. Sections of apple and pear stems from material 
fixed at different times of the year indicate that during seasons of 
much or of late radial growth cortical growth sometimes continues 
very late and is not completed by the time the period of dormancy 
arrives. The increase in diameter of wood necessitates and is followed 
by an increase in the area of the bark. When an adverse change in 
the weather conditions interferes before this cortical growth is com- 
pleted, the dormant period must be passed with the outer bark in 
this unfinished condition. In such a case the bark is often under 
considerably higher transverse tension than it is in cases in which its 
cortical growth has been finished. In instances also in which bark 
growth has been very slight during some years, the cell walls of the 
cortical tissues and those in the outer phloem are thickened to such 
an extent that a rather rapid resumption of radial growth is not 
immediately followed by cortical growth, and therefore high bark 
tension ensues. If such hardened outer bark is eventually forced 
into growth late in the season, some of the cells necessarily pass 
