124 



lackiiig and whose period of growth bae no definite limitation. 

 Escample^ of this second case are furnished by ivound-t issue 

 ±angus-ga;.is and others, ' 



^''^..^V^^T^-'^^ *^° ■g:^fe -history of hjrp^rplastic excrescences, 

 /i/reh V-f ^e4i.-aivision (producing thett will have to be tested first of 

 {135) all aB to thei-r direction. ;?e will find extensive tissue exerts- 

 cenoes, of which the celA walls show a perfectly regular arrange- 

 iiiejnt. Xn others a definite orientation ean be found only ±n the 

 tirst cell-divisions. When consid^i-ing galls, the difference 

 between ceil divisions parallel to the upper surface of the or- 

 gan » bearing the gall, and those perpendicular to it will be 

 proved worthy of attention. 



Fttrthey the tissue material, from which hyperplastic excres- 

 cenoes come, needs more exact testing from various points o'f view. 

 The excrescence may be traeed' either to meristematic tissue 03* 

 to peritanetit tissue. In the filrst case the direction of the di- 

 vision^ characteristic of the meristematic cell under normal ' ■ 

 conditions, must be compaired with that recognizable in normal 

 tissue formation. 



If the tissue excrescence arises from an organ or part of 

 an organ of which the tissues are already differentiated, the 

 further question arises as to whether all the different kinds of 

 tissues of a normal organ are equally o^dapted to the production 

 of abnormal tissues masses, it will be possible to determine 

 conformably in hyperplasias of different kinds that the cells of 

 the different tissues possess different degrees of capacity for 

 abnormal division; thus, for example, the epidermis remains con- 

 siderably below the mesophyll and bark tissues in its productive 

 power. 



(136) Finally the higtology of abnormal excrescences will have to 

 be studied. One of Culr chief tasks therefore will be a compari- 

 son of structures of a,bnormal tissue with the normal ones of its 

 ground tissue, i, e, if aeformations of the whole organs are eon- 

 cern$d, to compare the structure of the organs deformed hyper - 

 plastically with that of the corresponding normal ones. 



In all cases when judging of any tissue excrescence what- 

 ever, we will have to determine first of all, whether the abnor- 

 mal tissues resemble the normal, corresponding parts of the 

 plant, or whether they differ from them in a*iy way. This ques- 

 tion is of fundamental significance for our anatomical consider- 

 ations; indeed, we will divide all hyperplastic tissues into t<m~ 

 ooplastic and heteroplastic ones according to the answer found. 

 We will spe^k of an homooplasm if the cells produced in a tissue 

 excrescence resemble' the cells underlying them or adjacent to 

 them and of an heteroplas ia if the abnormal excrescences are com- 

 posed of cells differing ~frora the corresponding normal ones. 



Heteroplastic excrescences are of especial liistological in- 

 terest. As might have been expected from the beginning the dif- 

 ferences between normal tissue and the abnormal derivations, 'de- 

 viating from it in structure, are very diverse. For example, 

 there is offten a striking difference in size between normal and 

 abnormal cells. The difference in tissue-differentiation, how- 

 ever, is of greater importance. In very many cases the product 

 9f the heteropl^astic tissue formation is but little differentia- 

 ted. So far as the form of the separate cells and the differen- 

 tiation of the different tissue layers is concerned, we meet here 

 with primitive conditions simrlar to those found in hypopXasiia; 

 In oth^r cases, extensive differentiation occurs in the tissue, 

 which,- corresponding to the character of the heteroplaslas,-* 



