THE VITAL PROPERTIES OF THE CELL 119 



capillary tube is filled with *01 per cent, of malic ncid, and after its 

 surface has been most scrupulously cleansed, is reversed and care- 

 fully placed in a drop of water containing a large number of Fern 

 antherozoids. With a magnifying power of 100 to 200 diameters, 

 it can be seen that some antherozoids immediately begin to make 

 their way towards the opening of the tube, from which the malic 

 acid commences to diffuse itself throughout the water. They 

 soon force their way right into the tube itself, until after five or 

 ten minutes several hundreds of them have collected there. After 

 a short time there are only a few left outside of the tube. 



If experiments are made with solutions of malic acid of varying 

 strengths, a law similar to that of the effect produced by various 

 degrees of heat upon protoplasmic streaming movements may be 

 deduced. Beyond a certain minimum concentration {about "001 per 

 cent.) which may be considered to constitute the stimulative starting 

 point, every increase in concentration produces a corresponding in- 

 creased effect, until a certain fixed point is reached, when the optimum 

 or maximum result is produced; if the concentration is increased 

 above this point the attraction of the malic acid for the anthero- 

 zoids decreases, until finally the positive chemotropism is con- 

 verted into negative chemotropism. 



Hence a very strong solution produces an exactly opposite effect 

 to that produced by a weak one, the antherozoids being repelled 

 instead of attracted. How small a quantity of malic acid is 

 necessary to produce a result may be seen from the fact that in 

 a capillary tube which contains a *001 per cent, solution only 

 '0000000284 milligramme, or Y500V000 ^ a milligramme, of 

 malic acid is present. 



As has been already stated, if the chemical stimulus is to pro- 

 duce movements in a certain direction, it must only be strongly 

 applied at one point, or at any rate from one side. This is the 

 case in the above experiment, for as the malic acid becomes dif- 

 fused through the opening in the surrounding water, the anthero- 

 zoids, passing through the opening and making their way up the 

 tube, come into contact with solutions gradually increasing in 

 strength. The diffusion causes an unequal distribution of the 

 stimulus about the bodies of the antherozoids : " thus varying 

 with its varying degrees of concentration, the malic acid exerts a 

 stimulus which causes a movement in a fixed direction." 



The antherozoids, as might be expected, are distributed evenlj r 

 throughout a homogeneous solution, yet even under these condi- 



