Root Hairs and Growth
By obtaining such data the authors 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.
Abstract
WHY STUDY ROOT HAIRS? G OROWTH is the most complex of all biological processes. It is, indeed, a resultant of all of the physiological processes: absorption, synthesis, assimilation, conduction, digestion, respiration, and secretion. 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 physiological 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 determined by the resultant of the physiological 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, uch 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 considerations, they cannot go far in extending our