

Minnesota Plant Life. 499 



ance. They are not, however, the normal immediate responses 

 to the presence of certain temperature or light conditions and, 

 therefore, cannot be regarded as actually caused by light or heat, 

 as are certain induced movements. 



Among induced responses of living substance might be men- 

 tioned the change in position undergone by particles of leaf- 

 green when the illumination becomes intense. In many higher 

 plants the particles of leaf-green, so abundantly distributed 

 through the cells of the leaves, are shaped like two watch- 

 crystals faced together. That is, they are lens-shaped and 

 biconvex. When exposed to weaker illumination they custom- 

 arily turn their broad faces towards the light; but if the illum- 

 ination becomes too intense they shift their positions until only 

 their edges are presented to the rays. Another example of an 

 induced irritable movement is afforded by some free-swim- 

 ming cells that normally swim either towards a source of light 

 or away from it. Certain organisms of microscopic size are 

 known to swim invariably towards the positive or towards the 

 negative pole of an electric field. Still others are directed by 

 chemical substances in solution. Thus, the minute Euglena 

 plants swim towards concentration of organic substances that 

 would be suitable for food, and spermatozoids, directed by 

 chemical emanations, swim towards the eggs of their species 

 So invariably are these directive movements induced that if a 

 very weak solution of sugar, malic acid, or whatever the specific 

 chemical may be, is placed in a capillary glass tube and then 

 immersed in a dish of distilled water containing stimulable 

 spermatozoids, they will swim into the tube by thousands. 

 From such experiments it is learned that the spermatozoid finds 

 the egg with which it is to fuse, because it is induced to swim 

 from a weaker to a stronger solution of some substance that the 

 egg is excreting. 



Still another example of an induced movement is furnished 

 by the large jelly masses of living substance characteristic of 

 slime-moulds. These have a habit of climbing up upon the 

 bodies of herbs or other objects and spreading themselves out 

 on the leaves or elsewhere, as a preliminary to the production 

 of spores. The significance of such a habit is not hard to un- 

 derstand, for through it the spores when produced are likely 



