Polarity 119 



intercellular correlation? Once established, can it be reversed? Are the 

 physiological manifestations of polarity the cause or the results of mor- 

 phological polarity? Is polarity an aspect of the whole organism or do 

 individual cells possess it? 



To present the various phenomena of polar behavior and the problems 

 that they pose, it will be helpful to discuss the subject from several points 

 of view and to describe its manifestations in external structure, internal 

 structure, isolated cells and coenocytes, physiological activity, and the 

 development of organic pattern. 



POLARITY AS EXPRESSED IN EXTERNAL STRUCTURE 



The most conspicuous expression of polarity is in external morphology. 

 In higher plants the differences between root end and shoot end are de- 

 termined very early, perhaps at the first division of the fertilized egg. This 

 differentiation is not irreversible, however, for roots often appear on stems 

 under favorable conditions and, less commonly, buds and shoots appear 

 on roots. Polar behavior occurs in thallophytes and bryophytes, even in 

 some very simple forms like those of many filamentous algae, though 

 in such cases it is less sharply marked and more easily reversed than in 

 vascular plants. Organisms without morphological polarity are rare. A 

 few amoeboid forms have no axes in the vegetative stage but form 

 polarized fruiting bodies. Algae like Pleurococcus are spherical and 

 apparentlv apolar but may be induced to produce filaments, an expres- 

 sion of axiation. Forms like Spirogyra, desmids, and diatoms have an axis 

 of symmetry but its two poles seem to be alike. In most filamentous types, 

 however, a rhizoidal pole and a thallus pole can be distinguished. 



Experimentally, polarity can best be demonstrated through its ex- 

 pression in regeneration, and it is here that most of our information about 

 it has been gathered. Polar regeneration has long been known and 

 manipulated in the horticultural practices of vegetative reproduction. 



Vochting ( 1878 ) cut twigs of willow and kept them under moist con- 

 ditions. Some he left in their normal, upright orientation and others were 

 inverted. Regardless of orientation, however, roots tended to be re- 

 generated more vigorously from the morphologically basal end and shoots 

 from buds at the original apical end. This is the classical example of 

 polarity (Fig. 6-1). If such a shoot were cut into two or more parts 

 transversely, each part regenerated roots and shoots in the same polar 

 fashion. Even very short pieces of stem showed this polar character. 

 Vochting removed a ring of bark in the middle of a shoot and confirmed 

 earlier observations that roots were formed above the ring and shoots 

 below, just as if the stem had been cut in two. From these and similar 



