86; 



consequence of this a normal overgrowth edge (2 u) completely covered 

 with bark is produced which forms the outwardly noticeable circular wall 

 about the tip of the tuber (Fig. 204, ik). 



The fact, noticeable at first, that phloem fibres are found in the centre 

 of a wood body, leads to the conclusion that the tissue surrounding the 

 phloem fibre groups is the place where the formation of the wood begins. 

 This conclusion is still more strengthened by the structures near the tubers. 

 Frequently younger phloem bundles are found here, even at times the very 

 youngest ones just appearing from the cambial zone, which are surrounded 

 by peculiar, radially arranged cells (Fig. 204,5). In some cases these 

 plate-like cells of the "phloem circumvallation" turn blue with iodine and 

 sulfuric acid ; in most cases, however, they turn yellow. This shows that, 

 as a fact, the tissue surrounding the phloem group tends easily to cell 

 increase. 



The overgrowth of the phloem by cork tissue is in no way restricted to 

 the tissues surrounding the gnarl tuber. In the trees I have investigated it 

 was found in different places after many an injury. In this, however, the 

 cells always have the character of cork cells and serve excellently to cut off 

 a diseased phloem bundle from the healthy wood. Any one who has worked 

 much with diseased trees knows how sensitive the bark cells are which have 

 apparently so resistant a structure. Their brown color and the more dis- 

 tinct appearance of their layers make it possible to trace the disease deeper 

 into the healthy tissue than can be done in the surrounding bark parenchyma. 



The overgrowth of the phloem begins, as a rule, in the cells of the 

 phloem sheath and remains limited at times to one side, or at least develops 

 more vigorously on the outerside. Similar phenomena, like the overgrowth 

 of the phloem bundles, are found also in some parts of the parenchyma. 

 Without any reason, known as yet, the parenchyma here substitutes for the 

 core a meristem zone in the bark which increases by growing around the 

 centre of fibres, thus beginning the formation of bark tubers. Such tubers 

 have usually a som.ewhat regular structure since the course of the tissue 

 elements in several annual rings keeps to the same direction. In a median 

 longitudinal section which may be recognized by the fact that the medullary 

 rays lie in approximately the same plane, the bent vessels are cut through 

 their whole length so that they interrupt the dark, parallel wood cell zones 

 as clear, concentric rings. 



The drawings (Fig. 206) made from the bark of a healthy one-year-old 

 pear twig give an interesting contribution to the explanation of tuber forma- 

 tion. We see in Fig. 206, i, the basal part of a very strong one-year-old pear 

 shoot of which the buds (a) are set in the normal two-fifths position ; h is the 

 one-sided swelling in the centre of the internode, reproduced again in cross 

 section in Fig. 206, 5, cut through in the deepest part, which is turned 

 toward the base of the twig, in Fig. 206, 5 in the median region, and in Fig. 

 206, 4 in the highest zone. In Fig. 206, j, 4, 5, the same letters indicate the 

 same parts; r, the bark, g and g-, etc., the bark vascular bundles in various 



