THE USES OF CHEMOTAXIS AND ITS EXCITANTS 349 



fulfilled, as when the sperm is attracted to the ovum 1 . This latter is better 

 fulfilled by normal chemotaxis, although phobic reactions may also produce 

 movement to particular points. It is not always readily possible to dis- 

 tinguish between osmotactic and chemotactic irritability, especially when 

 repulsion only occurs with high concentration. If, however, an organism 

 responds only to very few substances, or to those in very great dilution, the 

 response is clearly due to the chemical properties of the exciting substance, 

 and not to any osmotactic action. The attraction of the antherozoids of 

 Ferns by malic acid, of Bacteria and flagellate Infusoria by peptone and 

 potassium salts, is undoubtedly a positively chemotactic response. 



Although many substances may stimulate a particular organism, a 

 special substance may exert a preponderating action, and may overpower 

 all others. In this sense malic acid or its salts may be regarded as the 

 special stimulating substance for the antherozoids of Ferns, and probably 

 cane-sugar for those of Mosses. The antherozoids of Hepaticae, Sphagna- 

 ceae and of Marsilia are either devoid of any chemotactic irritability or are 

 only very feebly sensitive, since no certain attraction has as yet been 

 observed with any substance or mixture of substances. It is worthy of note 

 that if such reacting organisms as Bacteria, Flagellatae, Volvocineae, the 

 zoospores of Saprolegnia and fungal hyphae are tabulated in descending 

 order according to the stimulating action of a substance upon them, 

 the order will on the whole follow approximately the same course when 

 another substance of similar constitution is used . 



The high sensitiveness to malic acid or to cane-sugar shown by the 

 antherozoids of Ferns and Mosses respectively does not involve any special 

 sensitiveness to peptone or potassium salts. These are in general the 

 strongest stimulatory substances for Bacteria, which respond but feebly to 

 malic acid and cane-sugar. Even in the case of Bacteria great differences 

 are shown, for certain forms are attracted by sulphuretted hydrogen, and one 

 species responds readily to dextrin which is usually but feebly chemotactic. 

 Similarly the antherozoids of Ferns are neither attracted nor repelled, 

 whereas very many bacteria, though not all, show pronounced chemotactic 

 response to this gas. Infusoria and Euglena are also aerotactic, although 

 they react but little (Infusoria) or not at all (Euglena) to other substances. 



It seems unlikely that the mode of perception of different substances 

 by a particular organism is in all cases the same ; that, for instance, the 

 primary reactions involved in a chemotactic response to acids or oxygen 

 are the same as when the response is due to the presence of peptone or 

 potassium salts. It may, however, with safety be concluded that the 

 development of a chemotactic irritability adapted to the perception of 



1 Since all nutrient materials do not act chemotropically, it is hardly advisable to follow Stahl 

 (Bot. Ztg., 1884, p. 165) and use the terms ' trophototropism ' and ' trophotaxis.' 



