CHAPgEH VI 227 



GElIEgAL OPSEHVATimiS OK THK KTIO:.OCY 131) DE-VELOBffilJ!]! OF 



PATHOLOGICAl P311KT n^ISSUJ^S. DISCUSSIONS OF CEl^ML 



Pi.THOLOGY. THEOKETICAL ViI.<2mR 



ffin the present, r.r\cl conolrdlng chapter -ye r;ill recapitulate 

 the results clxecAy n.ttalrecl, arranging then r.cco.-ding to dife-r- 

 ont general poirte of vieu. In this, opportunity \,-ill be offered 

 for rar.king reports on mcny inportcmt rat-'tters which at the tine 

 hr,d to he left unconsidered. 



The diversity of the forms disclosed hv tha study of plant 

 pathologir is extraordinarily large. '7e find among representa- 

 tives of the sane species, that leaves, which under normal con- 

 ditions, are developed into extensJve thin sheets of tissue, nay 

 under abnornal conditions occur in the for^ of slender insigni- 

 ficant scales, or sv>rell out to fleshy tissue cushions; instead 

 of a large leaf-hlade and a short petiole, vre find that a tiny 

 blade raay appear on an innensely lengthened stem, or the leaf 

 may be covered with very different kinds of st/ellings, or it raay 

 be transformed into a single massive lurap, v/hlch can be termed 

 "leaf" only boeausn of its position on the plant body and the 

 like. Hand in hand v^ith thefce macroscopically noticeable differ- 

 ences come variations in anatomical strrctui e. V/hile, in norm- 

 ally developed leaves of one and the same species, the same struc- 

 tural conditions are a:}.ways recognizable in cross and surface 

 sections, abnorrrKxl examples show Yer"^ great differences, accord- 

 ing to the nature of th<^ disease. The repertoire of structures 

 is inconoeivablj, extensive, especially in hyperplasias, produced 

 Jby the action of foreigri organisms on leaves end other organs. 

 The oapr.city for diverse and brightly colored forms possessed by 

 the cells which make up the leaves is astonishing;- a consider- 

 (E6T) ation of exclusively normal forms and structures would not lead 

 end to imagine such a diversity, T.^ recognize the fact that m 

 many coiirses are possible for the developmental progress of the 

 cells or cell groups and the puestion forces itself upon us;- 

 v/hat factors decide which one of the mtiny possibilities shall 

 ultimately be realized. It v/ill be necessary to ask, for example, 

 why the cells composing the primordial leaf, do not furnish 

 those derivatives in the process of division which we ar<?? accus- 

 tomed to find as normal components of "healthy" leaves;- or why , 

 in some cases, besides the normal cells, still others are pro- 

 duced, which have some abnormal characteristics. Obvioiisly the 

 action of variable factors is ref3.eated in these distinctly dif- 

 ferent forms, which factors act on definite kinds of cells and 

 thus influence the formative processes. Before taking up the 

 study of the factors at work and of their action, some general 

 remarks might be useful by way of introduction. 



Without doubt, processes and qualities noticed in organisms 

 and in portions of organisms are influenced by very different 

 kinds of factors, to a still greater degree, than are those of 

 inanimate bodies, Varioijs kinds of forces change the cells and 

 tissues in the most diverse ways. Mechanical pressure and 

 strain moTild the form differently, loss of water through dios- 

 mosis or evaporation decreases the volume, increases the concen- 

 tration of cell solutions containing water, changes the ormotic 

 pressure and so forth. Besides changes and Effects of this kind- 

 and similar ones, still others of unequally greater importance 

 come under consideration, which are not sufficiently e.^cplained 

 by the transversion of the amounts of energy supplied, ^ile 

 in the abpve mentioned processes of changes in energy 6q\ial 



