Polarity 133 



of wall polarities that determine growth in a number of directions. How 

 this is brought about is a morphogenetic problem at a different level 

 from most of those here discussed, and its solution may provide sugges- 

 tions for an approach to other problems of form. 



It is sometimes possible to demonstrate the polarity of single cells ex- 

 perimentally even though their contents are homogeneous and both ends 



Fig. 6-11. Various types of unequal and 

 polar cell divisions: I, in pollen grain; 

 II, in differentiation of root hairs in cer- 

 tain monocotyledons; III, in differentia- 

 tion of stomata in monocotyledons; 

 TV, in leaf cells of SpJiagnum; V, in for- 

 mation of sclereids in Monstera. (From 

 Biinning. ) 



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m 



appear to be alike. This can be done by isolating cells and observing the 

 structures that regenerate from them. Miehe (1905) accomplished this 

 in the filamentous alga Cladophora. Here polar organization is present 

 but not conspicuous. At the basal end is a rhizoid which attaches to the 

 substratum, and the rest of the filament or the thallus, a single row of 

 cells, is undifferentiated. Miehe plasmolyzed the cells of a filament just 



