5^9 



pare the cross-section given in Fig. 132, due to an artificially produced frost 

 split on the branch of an apple tree. Parallel with the appearance of the 

 first wood cells is that of the hard bast cells (Fig. 132 h h) in the bark. 

 These prosenchymatous elements in the edge of the wound, formed of 

 parenchymatous wood, are the initial stages of the normal annual ring for- 

 mation and extend from the edge of the wound backward, approaching one 

 another more and more closely, until they have united in a normal annual 

 ring on the healthy side. If we start with the normal annual ring zone on 

 the healthy side of the trunk, we may thus conceive this formation as 

 follows; it is as if the prosenchymatous tissue of a healthy annual ring 

 (Fig. 135 c h) had been divided into several radiating branches (Fig. 

 135 ^^) within the canker excrescence which chiefly consists of parenchyma 

 wood, rich in starch and containing here and there large crystals of calcium 

 oxalate. (Radial division of the annual ring.) 



The edges of the wound, themselves, are not found united; the cleft, 

 therefore, in spite of its narrowness, has never completely coalesced since 

 the outermost cells, edging the cleft, constantly die. 



In proportion to the uncommonly luxuriant new formation, the number 

 of dying cells in "closed canker" is very small. The dead place here always 

 forms only a narrow tv^isted cleft ; while in "open canker" the originally 

 dead tissue represents a broad surface and the dying back of the edges of 

 the wound extends so far that not only the wood surface which first remains 

 uncovered, but also each overgrowth edge is incompletely covered by the 

 succeeding one. 



The characteristic radial division, or splitting of an annual ring (Fig. 

 135 nh, h) within the woody, parenchymatous edges of the excrescence is 

 less conspicuous in open canker and may completely disappear in case the 

 entire trunk, which has remained healthy, participates, at the height of the 

 canker-wound, in the exorbitant thickening, i. e., excludes a one-sided hy- 

 pertrophy of the trunk. 



The determination of the dry substances in normal and cankerous 

 wood in the cherry gives a proof of the softness of the tissue in the canker 

 excrescence. Normal wood has 6O.9 per cent, of dry substances ; the 

 overlying canker wood, only 45.1 per cent. 



From the fact that the canker excrescence frequently exceeds consid- 

 erably the thickness of the two or three year old branch which bears it, we 

 may conclude that the excrescence which is never found on the green shoot 

 of the current year. i. e., begins only in the woody twig, must grow very 

 rapidly. With such rapid development of the tissue, it is not surprising 

 that the fluctuations between cloudy, wet weather and periods of drought 

 can so manifest themselves that, within one summer, alternate zones of 

 thin-walled and thick-walled wood are produced in the canker excrescence. 

 This is found if the darker zone, extending from the pith (m) in Fig. 135, 

 is traced further. It corresponds to the thick-walled wood elements and, 

 in the normal part of the trunk, indicates the autumn wood in contrast to 



