14 PLANT LIFE. 



parent, that in the absence of transitional forms, the 

 evolution of one from the other is difficult to prove. 



Recent experiments made with the spores of the Fucacem 

 or brown sea-weeds, for the purpose of determining the 

 relative influence of external agents in determining their 

 polarity, that is, the determination of those points on the 

 surface of the spore where the rhizoids and apical point, or 

 in popular language the root and stem, respectively first 

 appear on germination, revealed the following points. 

 Light determined the polarity of all spores experimented 

 upon except those of Fucus serrafus ; the apical point, that 

 directly develops into the thallus, always appearing on that 

 side of the spore on which the light fell, the rhizoids appear- 

 ing on the opposite and shaded side. In such cases the 

 orientation or direction of the plane of the first septum in 

 the germination of the spore is at right angles to the 

 incident ray of light. When the amount of light was 

 insufficient to induce polarity, it was found that a difference 

 in quantity of oxygen on the different sides of the spore 

 produced this effect, rhizoids developing from the side in 

 contact with the least amount of oxygen, the apical point 

 originating from the opposite side. Neither gravitation nor 

 contact with a solid body exercised any influence on the 

 polarity of the spores. The polarity of germinating Equisetum, 

 or Horsetail spores, can also be determined by light, the side 

 farthest away from the light producing the root-cell, that 

 nearest the source of light giving origin to the prothallium- 

 cell. In all the species mentioned the polarity could be 

 determined entirely by internal causes that could not be 

 formulated other than by stating them to be due to life, and 

 probably this last cause of polarity and orientation is the 

 one followed under normal conditions; that is, when 



