STEMS 751 



fragment. The gemmae of Marchantia develop chlorenchyma on which- 

 ever side is iUuminated, rhizoids appearing on the other side. Bryopsis 

 and some other algae, when reversed, develop shoots from the former 

 rhizoids, and rhizoids from the former shoots (figs. 1074, 1075), light 

 being regarded as the chief factor determining shoot formation; the 

 roots of Neottia and of Platycerium under similar conditions develop 

 into shoots. An inverted piece of a dandelion root develops shoots 

 from the end toward the root tip, if the other end is in water. Such 

 phenomena illustrate what has been termed a reversal of polarity. 



