THE GENERAL CHEMISTRY AND PHYSICS OF PLANT-LIFE. 259 



cause efforts have not been wanting to explain tlie growth-phe- 

 nomena in cell-life * from a purely physical basis. (See below.) 



A cell must have a certain deojree of turgor as a necessarv con- 

 dition of surface-growth ; hence turgor is a phenomenon always 

 accompanying surface-growth. Frequently the ratio of growth pro- 

 ceeds parallel with the turgor-force (de Vries). Onr knowledge of 

 turgor is, however, far from sufficient to give us a clear conception 

 of growth. There are certain substances known to physiological 

 chemistry which form vesicular deposits, the so-called membranes 

 of precipitation, as, for example, lime solution and tannin, sulphate 

 of copper and potassium ferro-cyanide. To these '• inorganic cells " 

 (vesicles) the "■ turgor-growth " theory is to a certain extent appli- 

 cable : taking up of water by endosmosis causes tlie artificial mem- 

 brane to expand and finally to rupture. At this rupture the solu- 

 tions within and without at once form a new membrane of precipi- 

 tation ; this may be repeated again and again. A cylindrical algal 

 cell, however, differs very materially from such artificial vesicles, 

 because it has a cellulose-membrane and plasmic utricle, and the 

 cell-wall can grow only with the aid of the plasm. In its chemical 

 nature the membrane is not merely a precipitate from the albumi- 

 nous substances and water. Continuing the comparison, one would 

 expect that the cylindrical cell would become nearly spherical in a 

 short time because of the equal expansion in all directions. Actually 

 it elongates in one direction, which indicates that a difference in the 

 expansion of the cell-wall in different directions is one of the con- 

 ditions of cell-growth. 



A. Zimmekmann' gives a brief summary of the efforts made by 

 differetit authors to give a mechanical explanation of the form and 

 position of cell- walls. We nxuBt estimate the work of Berthold and 

 Errera especially. I say " estimate," because it is very important 

 that we should not draw other conclusions than such as really 

 follow from the results of their investigations. According to Zim- 

 mermann, the following may be looked upon as being established by 

 the investigations of Berthold and Errera. 



It is an empirically derived rule rather than a generally estab- 

 lished fact that the cell-wall during cell-division begins as a surface 



' Berthold, Studien ilber Protoplasmamechanik, Leipzig, 1886.— Trans. 

 ' Beitrage zur Morpbologie und Physiologic der Pflauzenzelle, Tiibingen, 1891, 

 Heft 3. 



