STIMULATION AND ITS EESULTS 401 



stimuli there is evidence that vegetable protoplasm can 

 modify its normal behaviour in other ways when exposed 

 to similar influences. This form of sensitiveness is less 

 widely distributed than those which we have just discussed, 

 but instances of it are fairly abundant, especially among 

 the more lowly forms of plants. 



A certain number of unicellular organisms are strongly 

 affected by the presence of free oxygen. The most interest- 

 ing case of this sensitivity is that of Bacterium termo. 

 When several of these plants are placed in a drop of water 

 upon a slip of glass, covered, and examined under the micro- 

 scope, they are found to collect at the edge of the cover- 

 glass. If a small green Alga is placed in the drop of water 

 with them, and the slide exposed to light of sufficient 

 intensity to enable the decomposition of carbon dioxide to 

 take place, the coincident evolution of oxygen attracts the 

 bacteria, which at once swarm round the Alga. So sensi- 

 tive are they to this attraction, that if the spectrum of 

 sunlight is thrown upon the Alga, the bacteria accumulate 

 at those parts which are illuminated by the red and blue 

 rays, which we have seen to be capable of effecting the 

 exhalation of the oxygen. Kesponse to the attraction of 

 oxygen is not confined to these bacteria ; it is exhibited by 

 many zoospores and also by the plasmodia of some of the 

 Myxomycetes. 



When the necks of the archegonia of the BryopJiyta 

 and Pteridophyta open with a view to the fertilisation of 

 the oospheres which they contain, they discharge a certain 

 mucilaginous fluid, which attracts to the organ the free- 

 swimming antherozoids. Careful experiments have been 

 made in many cases to ascertain what is the nature of the 

 attraction, and it has been found that the mucilage contains 

 various substances which the antherozoids seek. In the 

 cases of the Ferns and some Selaginellas, it has been deter- 

 mined that . the attractive body is malic acid. When a 

 capillary tube containing a weak solution of this substance 

 is inserted into water containing some of the antherozoids, 



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