542 TRANSFORMATION OF ENERGY 



chemical substances, &c. Among these varied phenomena those of chemotaxis 

 have at present been undoubtedly studied most thoroughly (Pfeffer, 1884 and 

 1888), and hence it will be most appropriate to begin with them. 



One special example of chemotaxis, viz. aerotaxis, has long been known, 

 and we have already become acquainted with the directing influence of small 

 amounts of oxygen on motile Bacteria in discussing the elimination of oxygen 

 by green plants during carbon-dioxide assimilation. Bacteria react to many other 

 substances as well as to oxygen, seeking, so to speak, an optimum concentra- 

 tion of them and retreating from them when their concentrations are too high 

 or too low. Chemotactic movements are thus often purposeful, since they are the 

 means of bringing the organism under optimum vital conditions, but reactions 

 which are apparently purposeless are not unknown, reactions, for example, to 

 substances which the organism is not in the habit of encountering in nature, 

 and to which it is unable to adapt itself. Rothert (1901, p. 382) has observed 

 a case of this sort, where two Bacteria were attracted in a remarkable manner 

 by ether, a substance which could only be injurious to them. Again, note 

 must be taken of the fact that, as Pfeffer has observed, many chemotactic 

 organisms are often unconscious of the presence of such poisons as corrosive 

 sublimate or strychnine nitrate, while they run away from such injurious agents 

 as acids, alkalis, &c. Such exceptional behaviour, naturally, does not enter 

 into the question of the biological significance of chemotaxis, which we must 

 assume has always to do with the attainment of optimum environmental con- 

 ditions. Chemotaxis, however, fulfils a far more special role than this in relation 

 to the sexual cells of differentiated lower organisms and of higher plants also. 

 The marked chemotaxis of the male cells leads them to seek the female cell, and 

 fertilization is secured by the attraction exerted on the sperm by certain sub- 

 stances present in the ovum or excreted by it or by parts in its immediate 

 neighbourhood. It had been long suspected that the sperm did not reach the 

 ovum merely by chance but by the attractive influence of certain substances, 

 but exact proof of this was first afforded by Pfeffer (1884), who showed that it 

 was probable that, in the case of the ferns, malic acid excreted from the archego- 

 nium was the attractive agent in bringing the sperm into its vicinity. 



If antherozoids of ferns be observed under the microscope in a drop of 

 water, they will be seen to move generally in straight lines ; whenever they come 

 into the neighbourhood of an archegonium, however, they twist sharply round, 

 so as to direct their anterior ends towards the mouth of the archegonium ; they 

 thus rapidly approach it, enter its neck and fuse with the ovum in the interior. 

 In his experiments Pfeffer replaced the neck of the archegonium by a capillary 

 glass tube of about o-i mm. bore, which he filled with various substances, placing 

 it at the side of the cover-glass. When the tube was filled with a o-oi-o-5 per 

 cent, solution of malic acid, which was neutralized by appropriate means, Pfeffer 

 found that the sperms rushed towards it and entered it in great numbers. He 

 estimated that 60 sperms had managed to enter such a tube in 30 seconds and 

 600 of them in 5 minutes. If the movement of the sperms be retarded by using 

 a weak solution of gum it may be clearly seen that the sperms curve round 

 sharply the moment they come within the sphere of influence of the malic acid 

 diffusing out of the tube, and place their long axes parallel with the course of the 

 diffusion current. Without any acceleration of their movements they then 

 steer their way towards the more concentrated solution, straight for the opening 

 of the capillary tube. Since the sperms distribute themselves equally in a homo- 

 geneous solution of malic acid, just as they do in water, we are bound to regard 

 the unequal distribution of the acid as the directive stimulus. The fact that 

 numerous other substances have no power to induce such directive movements, 

 however, leads us to the conclusion that the diffusion current, as such, is not 

 the actual stimulus. 



