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SCIENCE. 



[N. S. Vol XII. No. 291. 



the upper portion becoming branches and those 

 from the lower portion becoming roots. The 

 primary or main shaft has two axes, a longi- 

 tudinal and a transverse. In endogenous plants 

 the longitudinal axis, although always existent, 

 is seldom visually well defined. In the woody 

 esogens, however, its position is clearly marked 

 by the central pith. The transverse axis is 

 visually inconspicuous in all plants but no 

 structural or functional portion has a more 

 real existence than has this axis. Its location 

 is in a discoid portion of the main shaft, and 

 from it the upward and downward growth- 

 forces diverge, or turn in opposite directions. 

 I have therefore called it the tropaxis. The 

 condition, or manifestation of growth-force, 

 which is normally exhibited by the part of the 

 plant above the tropaxis I have called apo- 

 tropism, and that exhibited by the part below 

 the tropaxis, epitropism, as explained in the 

 former article. Therefore while growth of the 

 respective parts is, in a general way, toward 

 and from the earth it may more distinctively be 

 said to proceed in opposite directions from the 

 tropaxis. 



Epitropism and apotropism reside potentially 

 in the individual cells of the growing parts of 

 the plant. Each condition is normal in its own 

 division and in the ordinary growth of the 

 plant each is stable as a physiological balance 

 to the other. Both epitropism and apotropism 

 are, however, less stable in some plants than 

 in others, in which cases the normal condition 

 is, at certain points, exchanged for the opposite 

 condition. That is, under circumstances pres- 

 ently to be mentioned, cells that are normally 

 apotropic change to an epitropic condition, when 

 secondary roots or aerial rootlets result ; and 

 under other circumstances epitropic cells un- 

 dergo the reverse change, when suckers or new 

 plants result. Again, there is a kind of both 

 epitropism and apotropism due to special physi- 

 ological causes. Therefore there are not less 

 than three kinds or grades of epitropism and 

 apotropism. 



The normal apotropic condition of that part 

 of the plant which is above the tropaxis may 

 be called primary apotropism. It is that mani- 

 festation of growth-force which is concerned in 

 giving form and character to all that part of 



the plant above ground. Secondary apotropism 

 is that condition which results in the change 

 of small clusters of cambium cells at certain 

 points upon the roots of a plant from their nor- 

 mal epitropic to a complete apotropic condition. 

 It is this change which results in the production 

 of suckers or new plants. Secondary apotropism 

 is sometimes spontaneous and sometimes due to 

 exciting causes, among which is the infliction 

 of wounds. Spontaneous results are seen in 

 the abundant suckers which rise from the roots 

 of the Silver-leaf poplar, and those caused by 

 wounds are seen in the suckers which freely 

 rise from the spade-wounded roots of the gar- 

 den cherry tree. Another interesting exam- 

 ple of secondary apotropism, which is ac- 

 companied by secondary epitropism, is seen 

 in the method of propagating willows and 

 Cottonwood trees which is sometimes practiced 

 on our prairie soils. Poles are cut down, 

 trimmed, notched at intervals with the ax, 

 and buried in furrows of moist earth. Clusters 

 of primary apotropic cambium cells adjacent 

 to those wounded by the ax take on secondary 

 apotropic action and suckers result, which are 

 well nourished in their early stage from the 

 poles. Special apotropism will be presently 

 mentioned. 



The three grades or kinds of epitropism 

 proper, are primary, secondary and special, all 

 of which are distinct from the ordinary epi- 

 tropism of gravitation. The latter is plainly 

 mechanical and is conspicuously observable in 

 the drooping of branches and in the downward 

 curving of the stems of heavy fruits. Primary 

 epitropism is confined to that part of the plant 

 below the tropaxis where it is a balancing, 

 but in some sense an opposing force to primary 

 apotropism. It is secondary epitropism which 

 is manifested in the production of secondary 

 roots and aerial rootlets. It may be spontaneous, 

 when it constitutes one of the acquired habits 

 of the plant in which it occurs, or it may be 

 due to accidental circumstances. In each case 

 clusters of apotropic cambium cells take on 

 epitropic action and form, not adventitious 

 buds, as is the normal habit of such cells, but 

 aerial rootlets or true roots. The aerial rootlets 

 of the ivy and the frequent rooting of creeping 

 plants are familiar examples of spontaneous 



