1920] HURD—FUCUS SPORES $3 



absorption of the filters, but also partly to the fact that they do 

 not all transmit the same number of wave lengths. 



Polarity 



The power of external factors to determine the polarity of a 

 germinating spore is, without doubt, the power to orient the 

 spindle of the first dividing nucleus, if, as in the ca^e of Fucus, that 

 polarity is established by the direction of the first cleavage plane. 

 The work on such orientations is very limited, and has often yielded 

 negative results. Drietsch established the polarity of sea-urchin 

 eggs by subjecting them to pressure, the spindle forming parallel 

 to the flattened sides of the egg. A number of investigators have 

 found that unilateral white light will establish the polarity of the 

 spores of some of the lower plants by causing the first cleavage 

 plane to be formed perpendicular to the direction of the incident 

 light. Without exception the cell on the darker side of the spore 

 becomes the rhizoidal cell, the other being apical. Equal illumina- 

 tion on all sides retards or prevents germination. This has been 

 demonstrated in Equisetum, Fucus, Ascophyllum, Pelvetia, Dictyota, 

 Laurencia, Cystoseira, Anthoceros, Fimbriaria, Gym no gramme, and 

 Puccinia. It has been proved that gravity and contact cannot 

 establish the polarity of these spores. 



The first report of this phenomenon of polarity established by 

 light is that of Staiil (20), who worked on Equisetum. H< found 

 that the first wall is formed perpendicular to light rays striking 

 the spores on one side only, and that if all sides are illuminated by 

 rotating the spores on a ciinostat, the formation of the wall is 

 retarded or prevented. The cell on the shaded side of the spore 

 becomes the rhizoidal cell. In darkness the formation of the 

 first wail follows no rule, and the rhizoids extend in every direction. 

 Staiil refers to earlier work on Marsilia and Chora which indicates 

 that gravity is a controlling factor in the orientation of the first 



division plane. 



Rosen vinge (19) showed that in Fucus spiralis there is no 

 relation between gravity and the first division plane, nor does 

 contact with a solid body have any effect. He got the same 

 orientation to light in Ascophyllum and Fucus that Stahl did 



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