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THE QUARTERLY REVIEW OF BIOLOGY 



understanding of the fundamental nature 

 of growth. In order to understand the 

 physiology of the plant or animal it is 

 necessary to place it under as nearly con- 

 stant conditions as possible, and then to 

 vary one condition and measure the effect 

 of this change upon one process as nearly 

 as possible. In like manner, in order to 

 study the growth of an organism inti- 

 mately, it is desirable to proceed in the 

 same way, noting the effect of the change 

 of one external condition upon one of the 

 cytological processes of cell division, cell 

 enlargement, and cell differentiation. 



The past fifty years have witnessed a 

 very intensive study of the mechanism of 

 cell division, beginning with the work of 

 Strasburger and Flemming. The study of 

 nuclear division has been especially thor- 

 oughly prosecuted, largely because of the 

 connection which has been established 

 between it and the study of the transmis- 

 sion of hereditary characters. But we are 

 still a long way from understanding how 

 hereditary factors bring about the charac- 

 ters of the organism during its individual 

 development, largely because we have 

 neglected the study of the other phases of 

 growth. We have not as yet determined 

 how it is that conditions determine the 

 plane of partition or the rate of cell 

 division, cell enlargement, or cell differen- 

 tiation. In order to do this it is desirable 

 that cells be chosen which undergo essen- 

 tially only one of these three processes to 

 the almost entire exclusion of the others, 

 and which can be studied while growing 

 normally under constant and controlled 

 conditions. 



In the study of cell division this ideal 

 can be almost realized by the use of bacteria 

 and other microorganisms in which there 

 is little differentiation or cell enlargement, 

 and in which the rate of cell division can 

 be measured by counting the number of 

 cells from time to time. But there is such 



a wide gap between the cell of the micro- 

 organisms and that of the higher plant or 

 animal that it is doubtful whether we are 

 at all justified in applying the conclusions 

 from the one to the interpretation of the 

 other. The nearest approach to a satis- 

 factory study of the effect of conditions 

 upon the rate of cell division in higher 

 organisms is by the method of tissue cul- 

 ture. This is, however, open to the 

 criticism that the cells doubtless behave 

 entirely differently in these cultures, than 

 they do in their normal relation in the 

 organism. 



A much more satisfactory study can be 

 made of the effect of external conditions 

 upon cell enlargement through a study 

 of root hairs. These are projections of 

 superficial cells of the roots of plants. 

 They are normally cylindrical in form, 

 and enlarge only by elongating. A meas- 

 urement of increase in length gives us a 

 means of directly calculating the increase 

 in volume of the cell, and hence its en- 

 largement. The root hairs are produced, 

 even, upon very young seedlings. This 

 makes it possible to study root hair elon- 

 gation while the entire plant at that stage 

 is intact. The root can be allowed to 

 extend into a moist chamber, or into a 

 vessel of solution on the stage of the 

 horizontal microscope. In this way we 

 can have the plant under normal and con- 

 trolled conditions and measure accurately 

 its response to different factors of the 

 environment in terms of cell enlargement, 

 withoutcomplicatingcircumstances. The 

 nucleus of the root hair does not divide, 

 neither is there any other evidence of cell 

 division occurring within the cell. The 

 process of cell differentiation is reduced 

 very nearly to a minimum. It here con- 

 sists of an extension of the cell wall and 

 an enlargement of the vacuoles incident to 

 cell enlargement. There is probably a 

 slight change in the composition of the 



