32 DURATION OF TQE SEVERAL MITOTIC STAGES 



A. RHYTHM IN MITOSIS, 

 (a). GENERAL. 



The beginning of the mitotic process in plants is conditioned upon 

 the state of cell-turgor, which in turn impHes that under conditions 

 normal to the growing tissue the cell has not only absorbed a definite 

 quantity of water, but also an amount of food materials and oxygen 

 sufficient to set up the necessary physical and chemical potential de- 

 manded, in the particular setting of things, to start the mitotic train. 



Strictly speaking, growth and mitosis are two distinct processes; 

 growth refers only to permanent increase in bulk; mitosis, on the other 

 hand, refers to cell-division regardless of increase or decrease in the size 

 of the end product. Not only are they distinct processes, but in the 

 same cell at the same time the one practically precludes the other. 

 But while mitosis and increase in bulk are different processes, they must 

 cooperate, if either is long to continue. Cells must divide, because 

 their contact with the external world is through their surfaces and is 

 therefore proportional to the square of their diameters; but their bulk 

 and consequently the amount of metabolic work they are called upon 

 to do vary with the cube of their diameters. A cell active mitotically 

 is resting from its normal metabolic activities; conversely, while a cell 

 is metabolically highly active it can not undergo mitosis. Sachs,^ in 

 his "Text-book of Botany," says: 



"This relation of growth, which is dependent on cell-division, to assimila- 

 tion, is especially clear in algae of simple structure (as Spirogyra, Vaucheria, 

 Hydrodictyon, Ulothrix, etc.), which assimilate in the daytime under the 

 influence of light, while cell-division proceeds exclusively or at least chiefly 

 at night 



"We have here a case of division of physiological work which shows us that 

 the cells which have to do with chemical work (assimilation) can not at the 

 same time perform the mechanical labor of cell-division ; the two kinds of labor 

 are distributed in the higher plants in space, in very simple plants in time. 

 Provided there is a supply of assimilated reserve-material, cell-division can 

 therefore take place either in the light or in the dark. Whether there are 

 special cases in which light promotes or hinders cell-division is not known 

 with certainty." 



Quoting Famintzin,^ Sachs continues: 



"The cell-division of Spirogyra has been proved to be dependent on light 

 to the same extent as the formation of starch ; but relationship in the former 

 case differs from that in the latter in the following respect : The formation of 

 starch is induced by a very brief exposure to light (about half-hour) and 

 requires that its action be direct; starch is formed only under the influence of 

 light ; in its absence the formation at once ceases. Cell-division, on the other 

 hand, is induced only after light has acted for some hours; it then commences 

 in the cells, whether these have been exposed to light for some time or have 

 been removed into the dark." 



1 Sachs, Julius, "Text-book of Botany." (Tr. by A. W. Bennett.) Ch. 3, pp. 659-689. 

 ^ Famintzin, Melanges phys. et chim. Petersbourg, 1868, Vol. III. 



