EXPERIMENTAL ANALYSIS 



Utilized by the cells.' Twenty years later Fischer^^ can only say: 'It 

 would be of great interest were we able to separate the forces acting in 

 preparing the division and those which act in releasing the division 

 process.' It is of importance, too, to distinguish between factors which 

 may initiate the process of mitosis and those influencing the division 

 rate under conditions where division would in any case occur. The 

 effects of suitable doses of radiations, e.g., heat (which may increase) 

 and ultraviolet (which may increase or decrease) division rate, are of 

 the second kind (Giese;!^ i* Loofbourow^^), 



There appear to be certain optimal dimensions to which a given type 

 of cell must approximate and, that being so, 'the question arises whether 

 the cells are induced to continue growing and consequently keep divid- 

 ing to maintain optimal dimensions or whether they are induced to 

 divide and must grow to maintain the same surface-volume ratios' 

 (Berrill^). Thompson^^ is committed only to the general statement 

 that 'the phenomenon of division of the growing cell, however it be 

 brought about, will be precisely what is wanted to keep fairly constant 

 the ratio between surface and mass, and to retain or restore the balance 

 between surface-energy and the other forces of the system', but Berrill^ 

 goes a little farther in concluding that 'cell proliferation is itself 

 primarily a response to changing surface relationships' and 'Cell 

 division must therefore occur when the cell size exceeds a certain critical 

 value.' To state this, however, only raises questions about how the cell 

 increases in size, and whether mere increase in size would really be a 

 sufficient stimulus to mitosis. The increase in size which is correlated 

 with mitosis is more probably itself merely one of the more obvious 

 manifestations of complex activities in which the whole cell and its 

 immediate environment are taking part. 



Plant growth substances 



In plants, as in animals, the phenomena usually grouped under the 

 name 'growth' are multiple. At the cellular level, cell enlargement, cell 

 division and cell organization may occur separately and independently 

 or simultaneously and perhaps interdependently. The plant 'growth 

 substances' of the auxin group — indolylacetic acid and its analogues, 

 appear to act, in low concentrations, on cell size only; at higher con- 

 centrations the principal response [e.g., in the rooting response of woody 

 cuttings) is cell division. At still higher concentrations, cell enlargement 

 and cell division may be so much accelerated that the tissues grow 

 faster than the roots can supply food to them. The result is necrosis, 

 and this mechanism accounts for the weed-killing properties of '2, 4-D'. 

 One of the first effects of auxin on the plant cell is to increase the rate 

 of cyclosis and possibly of the cell metabolism in general. This increased 

 cytoplasmic activity in the plant cell may well correspond to the 



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