THE INDUCTION OF POLARITY 



155 



and also most algae are bipolar (verticibasal), and this polarity is exhibited 

 even in the lateral shoots whose bases ancl apices differ more or less widely l . 

 As in the case of the transverse polarity known as dorsiventrality, it requires 

 to be determined in each case whether the similar or dissimilar polarity is 

 due to internal or external causes, and whether it is labile or stable in 

 character. 



In the equipotential apical 

 meristems of both shoot and 

 root it is the influence of the 

 preformed parts which de- 

 termines the mode of develop- 

 ment,independentlyof whether 

 a radial arrangement or a la- 

 bile or stable dorsiventrality 

 results. The cells of the pri- 

 mary meristem cannot possess 

 any inherent polarity, for if 

 they were endowed with a 

 fixed verticibasality like the 

 swarm- spores of Chlamido- 

 monciS) no such conversion of 

 the growing apex of a root 

 into that of a shoot could 

 take place as actually occurs. 

 That they possess no trans- 

 verse polarity is shown by the 

 fact that the same group of 

 cambium cells may according 

 to circumstances give rise to 

 the primordium of either a 

 shoot or a root. 



The differentiation and 

 division of labour for particular 

 functions involves a certain 

 fixation of polarity, although 

 this is not always externally 

 perceptible. It is best shown when a plant is cut into pieces, each of 

 which strives to replace the missing parts. For in both root and shoot, new 

 shoots tend to be formed at the normally upper end of each, and new roots 

 at the normally lower end, even when the pieces are hung upside down 

 (Fig. 26). Vochting has hence denoted the respective ends of such pieces 



B 



FIG. 26. Stems of Salix sprouting in darkness. They are sus- 

 pended by threads (_/"), A, in the normal, B, in the inverter) position. 

 Even in B the shoots are developed at the shoot-pole (s), the roots 

 at the root-pole (w). 



1 For details see Goebel, Organography, 1900, I, p. 64, and with regard to animals, Hertwig, 

 Lehrb. d. Zoologie, 1897, 4. Aufl., p. 107. 



