HYDROTROPISM— CHEMIOTAXIS 499 



sand, and is perforated so as to allow the rootlets to protrude, 

 these grow vertically downwards, according to tlieir geotropism. 

 As soon as they protrude, they curve to a greater or less extent 

 towards the moist surface, as if seeking the moisture. This 

 phenomenon is known as hijdrotrojpism. The root-tip, as in 

 other cases, is the sensitive part, while the curvature takes place 

 further back, where growth is most active. Negative hydro- 

 tropism is very rare, being exhibited only by some of the 

 Myxomycetes, which move away from moisture. 



The advantage of this form of sensibility is evident in the 

 case of the root, which, by virtue of it, is drawn towards the 

 moisture of the soil as it penetrates between its particles. 



A curious instance of appreciation of lack of moisture is 

 afforded hj Porlieria hyqrometrica, which under such conditions 

 closes its leaflets much as nyctitropic plants do when light gives 

 place to darkness. 



Chemical Stimuli. —Besides the forms of sensitiveness which 

 we have discussed so far, there is evidence that vegetable proto- 

 plasm can appreciate stimuli of a different kind. These are less 

 widely manifested, but instances are fairly abundant among the 

 lower forms of plants. 



A certain number of unicellular organisms are strongly 

 affected by the presence of free oxygen. The most interesting 

 case of this is that of Bacterium fermo; when a number of these 

 plants are placed in a drop of water on a slide, and examined 

 under the microscope, they are found to aggregate at the edges 

 of the cover- slip. If a small green Alga is placed in the drop 

 with them and the slide exposed to light of a sufficient inten- 

 sity to enable the decomposition of CO., to take place, the evolu- 

 tion of oxygen attracts the bacteria, which at once swarm round 

 the alga. So sensitive 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, the rays which we have seen to be capable of effecting 

 the decomposition of the CO.,. This response to the attraction 

 of oxygen is not confined to these bacteria ; it is afforded by 

 many zoospores, and also by the plasmodia of some of the 

 Myxomycetes. 



It has already been observed that when the necks of the 

 archegonia of the Cryptogams open fchey 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 



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