Oct. 14, 1918 Hardening Process in Plants 105 



DISCUSSION OF RESULTS 



Haberlandt (16) found that a growth similar to that of the tumor 

 cells of cabbage occurs in the storage cells which lie next to the phloem 

 tissue in small pieces cut from potato tubers. Growth occurs in pieces 

 0.5 by I by I mm. only when phloem cells are present. Xylem cells 

 are not necessary. Haberlandt stated that growth does not occur in 

 such cells because they are physiologically different from cells farther 

 from the phloem bundles. Neither is growth due to nutrient materials 

 coming from the phloem; nor is it due to the phloem cells giving off 

 substances which counteract the lethal overstimulation of the cells by 

 wound stimulus. He attributed the growth to the excretion of a growth. 

 producing enzym (Wuchsenzyme) by the phloem cells and stated that 

 the companion cells rather than the sieve tubes are responsible for it. 



In any such case or in freezing it is difficult to establish just what 

 wound stimulus consists in. In the case examined in this investigation 

 it appears that at least a part of the wound stimulus can be accounted 

 for by the change of the hydrogen-ion concentration on freezing and by 

 the consequent precipitation of proteins. This offers a tangible thing 

 to which wound stimulus can be ascribed. It is possible also that other 

 wound stimuli may be caused in a similar manner by desiccation. In 

 nearly all such cases it appears that a stimulus which causes growth on 

 moderate application will cause death on overstimulation of the cells. 

 Thus, the first layers of a cut surface of potato die, and phellogen forma- 

 tion is taken up by the lower layers. Similarly, cabbage cells are either 

 stimulated to growth or killed outright, according to the degree of freez- 

 ing. The greater stimulation of the tissue about the veins to form 

 tumors on freezing is possibly due to the fact that such cells are more 

 easily injured than the other cells of the leaf, and thus receive a stronger 

 stimulus. The easier precipitation of the proteins in juices expressed 

 from the midrib than from the rest of the leaf indicates such a physiologi- 

 cal difference. 



Cases of renewed growth or regeneration, such as shown by Klebs (19) 

 and Miehe (55) to be produced on plasmolysis, have usually been ascribed 

 to removal of the cells affected from the influence of those about them — 

 that is, to a removal of the correlation effect. It has been reported by 

 Klebs (ip) that growth after plasmolysis generally occurs only in the 

 algae. Miehe (jj) states that in such plasmolyzed cells only the living 

 continuity is destroyed. This statement is taken to refer to the plas- 

 modesmen. It has been shown by Gardiner (10) and others (11) that 

 there is a continuity of protoplasm between adjacent cells. Gardiner 

 (10) found that these threads of protoplasm were not broken in every 

 case during plasmolysis, but fine strands could be observed running out 

 to the cell wall. Mangham (25, 26) ascribes to the plasmodesmen an 

 important function in conducting sugar from cell to cell. Czapek (7) 



