392 VEGETABLE PHYSIOLOGY 



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 inter- 

 esting case of this sensibility is that of Bacterium termo ; 

 when a number of these plants are placed in a drop of 

 water upon a slip of glass 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. This response 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 Bryopliyta 

 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 

 determined 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, 

 they make their way very quickly to the orifice of the tube. 

 They are very sensitive to the presence of the acid, being 



