Polarity 121 



theory of polarity. The problem, however, is evidently not quite as simple 

 as Vochting at first thought. 



Polar regeneration is also evident in the lower groups of the plant 

 kingdom. If fern prothallia are sliced transversely, regeneration from 

 their cut surfaces is polar (Albaum, 1938fr; Fig. 6-2) and is related to 

 physiological gradients, especially of osmotic concentration (Gratzy- 

 Wardengg, 1929 ) . In isolated primary leaves of ferns, polarity is evident, 

 but both the character and the polar distribution of regenerated structures 

 are somewhat diverse (Beyerle, 1932). 



Fig. 6-2. Polar character of regeneration in fern prothallia from which pieces have 

 been removed by transverse cuts. An apical portion restores a single heart-shaped 

 structure, but from a basal one a group of small prothallia is formed. (From Albaum.) 



In the regeneration of hepatics and mosses, polar behavior varies. The 

 gemmae of Marchantia and Lunularia form rhizoids from either surface 

 while they are young but only from one when they grow older ( Haber- 

 landt, 1914), indicating that embryonic tissue, as it proves to be in many 

 other cases, is relatively unpolarized. Polarity here can be reversed by 

 gravity, light, and other environmental factors ( Fitting, 1938 ) . Vochting 

 found relatively little polarity in the regeneration of the thallus of 

 Marchantia. In the mosses, cuttings formed rhizoids at the lower end and 

 protonemata at the upper one. This behavior could be reversed by in- 

 version of the cuttings ( Westerdijk, 1907), but regenerating structures 

 were always more vigorous at the morphologically basal pole. 



Polarity is evident in the sporophores of the higher fungi but here, 

 also, it is not firmly fixed, for a segment of the pileus may be successfully 

 grafted back to the same pileus in an inverted position ( Lohwag, 1939 ) . 



In most algae there is a sharp distinction between the rhizoidal, or 

 hold-fast, pole and the thallus, or shoot, pole. Especially in the simple 

 forms and in early stages of the more complex ones, this polarity may be 

 reversed by changed relations to gravity, light, or other factors (Wulff, 

 1910; Zimmermann, 1923). Studies on the egg of Fucus and on coenocytic 

 algae are illuminating here (p. 135). 



Manifestations of polar behavior in higher plants are much more uni- 



