544 TRANSFORMATION OF ENERGY 



it was thought might be employed for determining the presence of mahc acid. 

 More recent investigations (Buller, 1900) have certainly introduced important 

 limitations. Not only malic acid but also a large number of organic and in- 

 organic salts operate attractively, but to all the non-dissociating bodies, such 

 as carbohydrates, glycerine, alcohol, asparagin, the sperms are quite indifferent. 

 The attractive power of the salt is manifested to the best advantage generally 

 when it is in a solution isosmotic with a o-i G.M. of potassium nitrate ; i G.M. 

 for the most part is injurious and o-oi G.M. is only rarely effective. Malates, 

 on the other hand, act in much weaker concentrations, from o-oi G.M., down to 

 o-oooi G.M., and free malic acid from o-ooi G.M. to o-oooi G.M. Since it is in 

 the highest degree improbable that solutions as concentrated as these are dis- 

 charged from the archegonium (that is to say equal to a i per cent, solution 

 of potassium nitrate, or of a 1-24 per cent, solution of potassium oxalate), it is 

 very likely, but not absolutely certain, that the antherozoids of ferns are at- 

 tracted by malic acid in the ovum itself, where it probably exists, as Pfeffer 

 has given us reason for believing, not as free malic acid but as a salt of that acid. 



As already remarked, there are also substances which act repulsively on 

 antherozoids. If the repulsion is brought about by high concentrations, as 

 in the case of malates, we have to deal rather with an osmotic than a chemical 

 influence, and hence we may draw a comparison between osmotaxis and chemo- 

 taxis (compare p. 547). Many substances, however, such as alcohol, acids, 

 alkalis, &c., doubtless repel owing to their chemical peculiarities, and, when 

 they do operate, they always do so repulsively only. Other substances, such 

 as free malic acid, attract when in weak concentrations (o-ooi G.M.) and repel 

 when in somewhat stronger concentration (o-i-o-oi G.M.). It is not yet clear 

 whether the individual ions operate in a different way, as Buller thinks. 



The antherozoids of Selaginella react to malic acid exactly in the same way 

 as do those of ferns, but the specific attractive stimulant has not as yet been 

 determined in the higher Pteridophyta ; although such a body is doubtless 

 present there also. [We now know that in all the Pteridophyta the chemotactic 

 stimulant is malic acid (Shibata, 1905). Still, interesting differences among 

 these have been discovered, for the sperms of Equisetum are sensitive to malic 

 acid only (Shibata, 1905 c), while those of the Filicinae (Shibata, 1905 b) respond 

 to maleic acid as well, but not to fumaric acid ; the antherozoids of Isoetes, 

 on the contrary, are sensitive to fumaric but not to maleic acid.] Among 

 Bryophyta we know as yet of such attractive chemicals only in the mosses. 

 The remarkable thing in this case is that the attractive substance appears to be 

 cane sugar, a non-dissociating compound. [Lidfors (1905) affirms that pro- 

 teids are the chemotactic agents in Marchantia.] The antherozoids of mosses 

 are in no sense inferior, so far as sensitivity is concerned, to those of ferns, for 

 the liminal stimulus intensity was determined by Pfeffer in Funaria as 

 a O-OOI per cent, for sugar solution. 



Chemotactic movements have also been recognized in Bacteria, Flagellata, 

 and in the swarmspores of Saprolegnia (Pfeffer, 1888 ; Stance, 1890), as the 

 result of the action of various, but not all, materials nutrient to these organisms. 

 Among inorganic bodies, potassium salts and phosphates are effective, among 

 organic substances, peptone and asparagin but not glycerine. At the begin- 

 ning of this lecture we drew attention to the fact that oxygen had also a 

 chemotactic effect ; we may now add that it acts in a strongly repulsive 

 manner to certain anaerobes, although, it must be remembered, other bodies 

 also act in the same way. Many specific differences may be noticed in this 

 relation ; certain sulphur-Bacteria, for instance, are attracted by sulphuretted 

 hydrogen, a substance which, under ordinary circumstances, has never that 

 effect on ordinary motile cells (Miyoshi, 1897). Weber's law, as might indeed 

 have been expected, has been found to apply to certain other cases also. There 



