8i3 



and in the cross section (Fig. 191 u). Except for the folding of the bark 

 body no perceptible uniformly increased thickening is seen in the wood. 



In the apple branch here drawn the proportion of thickness between 

 the bark on the under side and the upper side is 50 to 42, while that on the 

 under side of the wood is 2 to i.- The pith (w) seems in the longitudinal 

 section slightly brown in stripes, especially in the lower half. Under the 

 microscope many of the cells of the pith and the pith crown, often arranged 

 in wavy lines, are found to have a brownish content and browned walls 

 which, in various cells belonging to the under side of the pith, are sharply 

 bent here and there and at these places separated from one another by 

 newly produced intercellular spaces (Fig. 194). The cells show the same 

 separation even in the cross section. 



The disturbances to the bark may be recognized most easily in the 

 projecting folds of the under side (Figs. 190 and 191, rf). In such folds, 

 split off from the wood by the bending, the phloem bundles (Fig. 192, hb), 

 as a rule, show a marked outward curving, corresponding to the peripheral 

 cork layers (k) produced in considerable thickness by the squeezing of the 

 epidermal cells and corresponding also 

 to the bark parenchyma (r), which 

 has been broken up by numerous holes 

 (/) into irregular particles. Some 

 time after bending some bridges of 

 radially elongated cell rows are found 

 in these holes, produced by the elon- 

 gation of the still elastic cells of the 

 young, inner bark. 



The apple branch in question was 

 bent at the beginning of summer as is generally done in practice. The bark 

 has been pushed up from the wood at the above described folds in the cam- 

 bial region. The relief from bark pressure at these places has resulted in 

 the formation of an abundant wood parenchyma, filled with starch, as shown 

 in the longitudinal section through the wood (Fig. 193 hp). After the holes 

 have been filled in and the bark pressure re-established, the wood paren- 

 chyma has gradually changed into normal wood (Fig. 193 hh'). 



The filling of the holes takes place here after the coalescence of both 

 parenchyma parts growing toward one another and uniting in the medial 

 zone (2). This yellow colored zone, under strong magnification, resolves 

 itself into a stripe of closely compressed cells. In other cases, the filling of 

 the holes is produced also by new parenchymatous structures from the raised 

 bark zone as well as from the remaining young sapwood tissue (as in bark 

 wounds). In all cases vessels first begin to appear in the wood parenchyma 

 after the holes are filled out; they gradually reach their normal length and 

 development, are accompanied at first by shorter, thinner-walled wood cells, 

 later by normally long and thicker-walled ones, and thus the normal wood 

 formation begins. 



Fig-. 194. a pith cells which have been 



broken apart in the bending-; b those 



which have remained uninjured. 



