262 



There were also centres of disease, which remained restricted to definite 

 groups of tissues. 



When one of the browned spots, covered with longitudinal bands due to 

 the darker veining, was cut through, it was found that its paper-like consis- 

 tency was not produced by a possible atrophy of the tissues, resulting from 

 an injury due to insects, or from bacteriosis, but only by the drying 

 together of the mesophyll cells, which have been almost entirely depleted of 

 their contents. The boundary between the dead and the wall-like convex 

 bordering healthy tissue was sharp, with no transitions. The collapsed brown 

 or (mostly) light walled tissue when treated with iodine, showed only iso- 

 lated flakes of cytoplasmic contents together with little drops of a colorless 

 or golden-yellow substance. With the entrance of water, the cell walls, like 

 the folds of an accordion, were raised somewhat from one another, without 

 the cells having been brought to their previous size. In the absolutely dead 

 tissue isolated, colorless, slender mycelial threads were found at times. 



If glycerine was allowed to act on the fresh sections, which, moreover, 

 also gives a strong acid reaction at the diseased spots and shows no oxydases 

 and peroxydases with guaiak and hydrogen peroxid, large, irregular or 

 usually spherical masses were drawn together in the cell contents. This phe- 

 nomenon was often found in especially sappy tissue, rich in sugar. At the 

 periphery of these masses lay the chloroplasts. In the badly diseased parts 

 these groups of substances could not be found at all, but only numerous very 

 small or somewhat larger drops. Just as Httle can this contraction of the 

 cell contents into strongly refractive drops be proved in the healthy part of 

 the leaf. We might place it in the list of glucoses because, with the Trom- 

 mer test, they show in places precipitates of cuprous oxid. 



Further anatomical investigations led to the discovery that, in the var- 

 ious yellowish tissue centres, the cell content was used up too strongly, and 

 the mesophyll cells had grown out wider. The diseased place thus became 

 somewhat swollen up over the healthy surface, but at once the diseased tissue, 

 which had lived out its life very rapidly, showed this by the appearance of 

 carotin drops ; it collapsed, turned brown, and dried up. This process of 

 drying, however, is limited, in all cases observed as yet, to the leaf region 

 characterised in the beginning by the turning yellow. In this the phenome- 

 non is distinguished from fungous infections. Since now enormously in- 

 creased formation of sugar can be proved and the absence of parasites de- 

 termined in the majority of spots, we have under consideration a constitu- 

 tional disease which set in, where the orchids named were cultivated in leaf 

 mould. 



This cultural method has been especially recommended in the last few 

 years by Belgian and English gardeners and introduced into Germany in 

 part with the use of Flemish leaf mould. After the rapid spread of the dis- 

 ease, the old process of growing the plants in a mixture of sphagnum with 

 bits of moor soil was again followed and the earlier results were again ob- 



