ROOT HAIRS AND GROWTH 



By CLIFFORD H. FARR 



Henry Shaw School of Botany, Washington University, St. Louis, Mo. 



WHY STUDY ROOT HAIRS? 



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ROWTH is the most complex 

 of all biological processes. 

 It is, indeed, a resultant of all 

 of the physiological processes : 

 absorption, synthesis, assimilation, con- 

 duction, digestion, respiration, and secre- 

 tion. Any circumstance which affects any 

 one of these physiological processes is 

 likely to affect the rate or the type of 

 growth. Thus the increased absorption 

 of a substance may increase the rate of 

 growth; or, on the other hand, it may 

 retard growth, according to the kind of 

 substance and the state of the organism. 

 In general if the processes that tend to 

 build up the organism, such as synthesis 

 and assimilation, occur more rapidly than 

 the processes which tend to tear it down, 

 such as digestion and respiration, growth 

 will occur. Growth may therefore be 

 looked upon as the net result of physio- 

 logical activity. It is a sort of index of 

 the well being of the organism as a whole. 

 Growth however is not to be looked 

 upon as the sum of all of the physiological 

 processes. They are to be regarded more 

 or less as aspects of the activity of the 

 organism as a whole. Growth on the 

 other hand is fundamentally a cellular 

 phenomenon. While its rate is deter- 

 mined by the resultant of the physiologi- 

 cal processes, it is itself a combination 

 of cytological processes. It embraces, in 

 fact, three phases of cell activity: division, 



enlargement, and differentiation. Cells 

 undergo development in three ways. 

 They divide, they enlarge and they 

 differentiate. By differentiation is meant 

 the change of an embryonic cell into a 

 tissue cell. This change may consist in 

 an alteration of form, composition, or 

 content. It may mean the appearance of 

 new organs, such as plastids, vacuoles, 

 and so forth. It may, on the other hand, 

 mean the disappearance of the nucleus, as 

 in certain of the blood corpuscles or in 

 sieve tubes; or the entire protoplasm of 

 the cell may die and decompose, leaving 

 only the thickened cell wall, as in certain 

 of the plant fibers or conductive vessels. 



We may, it is true, determine the growth 

 of an organism by determining its increase 

 in size or weight. Especially in the case 

 of plants the increase in size or weight 

 of only a portion of the organism, as the 

 fruit or tops, may be taken; or it may be 

 only of a portion of the constituents, such 

 as dry matter or ash. By obtaining such 

 data we can ascertain the effect of certain 

 external conditions upon the state of the 

 organism as a whole, without analyzing 

 its effect, as to whether it stimulates or 

 retards absorption or assimilation, etc., 

 or as to how it affects cell division or 

 enlargement or differentiation. 



It is apparent that while such studies 

 are useful in increasing our knowledge of 

 the effect of external conditions upon crop 

 production and such practical considera- 

 tions, they cannot go far in extending our 



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