Polarity 137 



has only one large nucleus, situated usually at the base of the stalk. 

 Here the rhizoids develop. At the summit of the stalk is the disk. If a 

 nucleus is introduced into a plant or plant segment which lacks one, a 

 new rhizoid system will arise wherever the new nucleus is placed and 

 polar behavior of the plant may thus be modified or reversed ( Hammer- 

 ling, 1955). 



In the germination of a moss spore, the young protonema pushes out on 

 the side of the spore toward the light, and the rhizoid forms at the op- 

 posite end, indicating that here, as in the Fucus egg, its polarity is de- 

 termined by light. In several moss species, Fitting (1949) was able to re- 

 verse this polarity by reversing the direction of the light. In this way the 

 young protonema becomes converted into a rhizoid. 



Heitz ( 1940 ) prevented polar germination of Funaria spores by appli- 

 cation of auxin. Cell division was also inhibited by this means and 

 "giant" cells thus produced. D. von Wettstein (1953) confirmed this and 

 found that vitamin Bi and chloral hydrate destroyed polarity without 

 preventing cell division. Such apolar growth continued for 50 cell genera- 

 tions, producing an undifferentiated, tumor-like body. Such a result 

 emphasizes the importance of polar behavior for orderly development 

 and the production of form. 



How the polar axis is determined in the spores of vascular plants has 

 been demonstrated in a few cases. In Equisetwn, the spore of which 

 shows no external or internal polarity, germination is followed by di- 

 vision into two cells. The division wall, as in Fucus, is laid down at right 

 angles to the gradient of light absorption (Stahl, 1885). The more strongly 

 illuminated daughter cell becomes the primary prothallial cell and the 

 one on the darker side, the rhizoidal cell. Nienburg (1924) showed that 

 this alignment of the mitotic figure parallel to the direction of the inci- 

 dence of light does not occur until a redistribution of cytoplasmic ma- 

 terial has taken place, especially an aggregation of chloroplasts on the 

 illuminated side (Fig. 6-14). 



In germinating fern spores, light modifies the polar behavior but this 

 effect is different in different wave lengths (Mohr, 1956). Naf (1953), 

 also working with ferns, found evidence that the spore of Onoclea has an 

 inherent polarity but that this can be modified by light. He carried the 

 study of polarity reversal much further by growing the young prothallia 

 in a liquid culture which was constantly shaken. The prothallia thus 

 developed in an environment where there were no environmental 

 gradients and where the plant was exposed on all sides to equal stimu- 

 lation by gravity, light, and other factors. The result was a spherical, 

 tumor-like mass of tissue. Grown on agar and without movement, this 

 tissue again formed structures much like the normal prothallia. The 

 genetic basis for a normal prothallium, specific in character, is in the 



