30 



Secondly, chemical changes, which the cell wall 1ft 

 many dases, undergoes in the course of its development, 

 especiajtly in a process of lignifi cation, must be consid- 

 ered. While the grov;th in thickness of the cell wall may 

 be arrested in different plants, and by rerir different kinds 

 of disturbing influences, the cases are rare in which thick- 

 BBed cell walls, like those of the sclerenchyma, the ducts, 

 etc, are excluded from lignifioation. Examples are fur- 

 nished by the Crataegus branches infected with Roestelia, of 

 which the medullarjr parenchj/ma remains unlignified (Wakker 

 loc. cit.), the Raphanus shoots attacked by Cj^stopus, whose 

 ducts remain unlignified, etc.-^ It is remarkable that even 

 under the life conditions, which are furnished our fruit 

 trees under cultivation, the lignifying process may be omit- 

 ted. Perhaps abundant water supplv and excessive nutrition 

 are the decisive factors. Sorauer^ found partially unligni- 

 fied pith in the fruit spurs. 



Finally, the phenomena of reabsorption must be consid- 

 ered, which under normal conditions, occur in the membranes 

 of many cells and lead to the building of the so-called 

 cell fusions, for instance in the sieve tubes. Inder nor- 

 mal conditions this reabsorption can be omitted, for exam- 

 ple, only tracheids maybe developed instead of ducts. 

 Since, under the action of unfavorable life conditions, 

 the width of the lumen of the ducts decreases greatly, it 

 is not always easy to give information concerning the occur- 

 rence or the omission of this fusion. Wakker found that 

 reabsorption was omitted in different plants v/hich were 

 infected with fungi (Vaccinium with Exobasidiuiji, Crataegus 

 \vith Roestelia, Rhamnus with Aecidium). Doubtless, in 

 etiolated leaves and stems and in individuals v/hich have 

 matured with arrested transpiration, the same arrestment 

 may be demonstrated in the formation of the elements vi^hich 

 convey water. 



Finally under the influence of arresting factors, the 

 formation of cross walls may remain so incomplete, that 

 instead of separate spaces, chambers may arise communicat- 

 ing with one another, or in the end, the formation of the 

 cross walls is entirely omitted. If the cells continue their 

 growth simultaneously, abnormally Isrge cells are produced, 

 and if their miclei also divide at the same time they result 

 (35) in multinucleated cells of abnormal capacity* For the 



present, we v/ill be content with a brief reference to arrest- 

 ed developments of this kind. On account of the external 

 conformity existing between cells of abnormal size produced 

 by arrestment of the moirbrane formation, and those of abnof- 

 mal size developed by abnormal growth (hypertrophy) we will 

 postpone their closer discussion to the fourth chapter. 



In what wajr arresting factors influence the de- 

 velopment of cross walls has been ascertained as yet only 



■^ Peglion, Studio anat. di als. impertrofie indotte dal 

 Cystopus candidus in als. org. di Raphanus Raphanistrum. 

 Riv. pat. veg. 1892, Vol. I, p. 265. 



2 

 jj Nachweis der Verweichlichung der Zweige unserer Obst- 

 baume durch die Kultur, Zeitschr. f. Pflanzenkrankh. , 1892, 

 Bd. II, p. 66, 143. 



