260 COMPENDIUM OF GENERAL BOTANY. 



of smallest area. Although the young cell-aggregates resemble the 

 vesicles of soap-suds in their arrangement and in the position of the 

 walls, 1 yet it must not be assumed that this offers a mechanical ex- 

 planation of cell-wall formation. The attempt to explain the me- 

 chanics of the exceptions to the rule seems especially futile. Further, 

 every anatomist knows that in the development of plants and plant- 

 organs we are not only concerned with cell-aggregates which are 

 divided by surfaces of least area: the cell- walls are intimately cor- 

 related to the form of the organs as well as to the ultimate function 

 of the cells. All that we can comprehend of this correlation is that 

 it serves a specific purpose. Berthold himself does not give an 

 exact mechanical explanation of the arrangement of cell-walls. 



The growing cell-wall (for example, of Spirogyra) can not be 

 compared to a liquid layer, as EERERA has done ; the only resem- 

 blance is that of form. ZIMMEEMANN summarizes NAGELI and 

 SCHWENDENEE'S (Microskop) explanations of the causes of the cell- 

 forms. According to these authors each cell and each cell-com- 

 plex has a tendency to assume a spherical form due to hydrostatic 

 pressure. Although this is in harmony with the view that the cell- 

 walls form surfaces of smallest area, yet the authors did not believe 

 that they had discovered a fundamental principle of the mechanics 

 of plasm. If we consider how various the growth-processes of 

 a cell are, we will refrain from expressing the opinion that the ma- 

 jority of growth-phenomena can be explained mechanically. In 

 one case the cell grows in length, in another it expands into an 

 oogonium ; again, it branches ; here it may grow and not divide ; it 

 may divide and not grow ; again, it may grow in thickness only, 

 either locally or uniformly ; etc. We have not a thorough under- 

 standing of a single phenomenon of growth. 2 



Although there is no satisfactory explanation of -growth, and no 

 mechanics of plasm, there is a physiology of growth. We shall 

 briefly mention the more important facts in regard to it. 



1 PLATEAU'S "Gleicbgewichtsfiguren." 



2 SCHWENDENER made the following important statements : " He who en- 

 deavors to solve some definite problem and who in the course of his investigations 

 meets with insurmountable difficulties has at least found a valuable insight into 

 his work, and his fellow-workers will be much indebted to him if he makes known 

 his experience. But he who does not see the existing difficulties and who be- 

 lieves he has found the final explanation when in reality only misunderstood 

 processes are described, tends to confuse the mechanical-physical investigation 

 rather than to promote it." (Rectoratsrede, Berlin, 1887.) 



