GROWTH AND MOVEMENT 441 



the embryo initial from the other), it is assumed that the two hemi- 

 spheres are unlike, even though no structural differences are visible. 

 This is expressed by the term polarity, after the analogy of the invisible 

 differences in the two ends or poles of the magnet. A like polarity must 

 be imputed to all other cells, its progeny, so that the embryo initial, when 

 it develops, produces at the one end a root and at the other a shoot. 

 Later in life, any piece of the shoot cut away from the rest shows a ten- 

 dency to produce shoots at the apical end and roots at the basal end, when 

 put under conditions to regenerate lost organs. The conception of 

 polarity in the cells is thus extended to aggregates of cells of any size, 

 because they show such differences at the apical and basal ends. All 

 attempts to ascertain the nature of polarity have so far proved futile, 

 so that there is nothing to "explain " the phenomena but the word and 

 the assumption for which it stands. 



Correlations. — The term correlation designates the reciprocal influ- 

 ence of organs. Of this little is known beyond the fact that the suppres- 

 sion or the removal of one organ exercises a marked effect upon some or 

 all of the remaining ones. Many examples might be cited, but no ade- 

 quate explanation of the effects can be given. It is known, however, 

 that at least some of them are not due merely to differences in the food 

 or water supply, or to like conditions. Examples will make clear what 

 is meant by correlations. 



Quantitative correlations. — In the axil of each cotyledon of the bean there is 

 present a bud, neither of which develops into a shoot unless the main axis is cut 

 off or prevented from developing. If one desires sweet peas and such plants to 

 continue flowering, it is necessary to cut away the older flowers or the young pods, 

 so as to prevent the formation of fruit. If this is done, the plants go on flowering 

 till frost, whereas their season is quickly over when allowed to set seed. The 

 gametophyte of ferns is short-lived, as a rule ; but if the fertilization of the egg 

 be prevented, its life may be prolonged for months, and it proliferates, forming 

 archegonia again and again. The possibility of shaping a tree by judicious prun- 

 ing, and of increasing the production of fruits by orchard trees in the same way, 

 rests upon like reactions. 



Qualitative correlations. — Correlations are not merely quantitative, as the above 

 examples might seem to imply; they arc also qualitative. That is, the whole be- 

 havior and even the structure of an organ may be altered according as other organs 

 are present or absent. Thus, the central axis of most conifers is strictly radial in 

 structure and in branching, while the lateral brandies are distinctly dorsiventral. 

 But if the terminal shoot be cut away, one (or more) of the laterals may become 

 erect, losing entirely the dorsiventrality, and becoming radial like the leader. The 

 aerial shoots of the potato, which bear foliage leaves and (lowers, are very different 

 from the subterranean ones, which bear the scales and tubers. But if the aerial 



