385 



cates that it will curve outward like a cap in the mature tissue of 

 the excrescence. 



This conical elevation of the cambial zone is visible in Fig. 64 wc. 

 It forms also an apical region, which, however, does not lie at the outermost 

 tip of the excrescence but always remains covered by bark tissue, which 

 dies from the outside inward until it reaches the meristematic tip of the 

 excrescence cone. 



The apical as well as the basal region of the meristematic zone of the 

 gnarl cone begins to develop shoots in the following year. Successful 

 sections, showing the full course of a medullar}^ ray, demonstrate that the 

 formation of the secondary axes takes place repeatedly in the same way in 

 which the priman,^ gnarl cone was produced, — viz., by the outgrowth of 

 part of the medullary ray extending through the bark. 



If the structure of the internodes is traced from the spot already 

 recognizable as the primordium of a gnarl towards the younger parts of 

 the branch, a lack of uniformity in the structure of the medullary rays is 

 seen in the very weakly developed wood ring of the axis. At the base of 

 the buds of the current year in which the immature wood cylinder has only 

 the spiral ducts of the pith crown and a few libriform fibres, together with 

 scattered, reticulated or porous ducts, medullary rays may be found here 

 and there which vary from the other rays in the somewhat greater width 

 of the cells, the somewhat stronger refractive power of the cell walls, the 

 distinct straight course and the further continuation in the bark. It is 

 noteworthy here that the end of the phloem ray extending furthermost 

 into the bark, unlike the other phloem rays, is not more slender than those 

 liehind it. but broader, in fact, the broadest of all the cells composing the 

 ray. While, therefore, the normal medullary rays are conical, this one has 

 turned its broadest base toward the periphery. This is the same tendency 

 in growth, found in the older stages, which appear as distinct excrescence 

 rays. Such a differentiation in the earliest stage shows how this goitre 

 gnarl formation is prepared in the first juvenile phases of the axis. 



Besides the excrescences of the medullary rays, there are still other 

 factors which distend the bark during the encysting of diseased tissue 

 centres. We will return to these points in the section on the "tuber gnarls" 

 which are best treated under the processes of wound heaUng. 



I had an opportunity to obsen^e in Primus Pad us the formation of 

 goitre gnarls, which branch like witches broom, and have found similar 

 structures on gooseberries^ I also found warty gnarls, similar to those 

 described in Ribes, in Cydonia vulgaris^. On gooseberry bushes near com- 

 post heaps, I could later determine gnarl structures in a form similar to those 

 in the black currant^ In a case in the red cherry currant, of which I heard 

 only recently, long leafy shoots which had no mature buds on their leaf 



1 Jahresbericht des Sonderausschusses fiir Pflanzenschutz. Arb. d. Deutsch. 

 L.andw.-Ges. 1898, p. 145. 



2 Ibid. 1899, p. 188. 



3 Ibid. 1900, p. 213. 



