BIOLOGY IN RELATION TO OTHER NATURAL SCIENCES. 457 



carbon and conseqnent diseugagemewt of oxygen is most active in the 

 part of the filament which receives the red rays (B to C). 



To this part therefore Avhere there is a dark band of absori^tion, 

 the bacteria which want oxygen are attracted in crowds. The motive 

 which brings them together is their desire for oxygen. Let us compare 

 other instances in wliich the source of attraction is food. 



The Plasmodia of tlie Myxomycetes, particularly one which has been 

 recently investigated by Mr. Arthur Lister,* may be taken as a typical 

 instance of what may be called the chemical allurement of liviug proto- 

 plasm. In this organism, which in the active state is an expansion of 

 labile living material, the delicacy of the reaction is comparable to that 

 of the sense of smell in those animals in which the olfactory organs are 

 adapted to an aquatic life. Just as for example the dogfish is attracted 

 by food which it can not see, so the Plasmodium of Budhamia becomes 

 aware, as' if it smelled it, of the presence of its food — a particular kind 

 of fungus. I have no diagram to explain this, but will ask you to 

 imagine an expansion of living material, quite structureless, spreading 

 itself along a wet surface; that this expansion of transparent material 

 is bounded by an irregular coast line; and that somewhere near the 

 coast there has been placed a fragment of the material on which the 

 Badhamia feeds. The presence of this bit of Stereum produces an 

 excitement at the part of the Plasmodium next to it. Towards this 

 center of activity streams of living material converge. Soon the afliux 

 leads to an outgrowth of the Plasmodium, which in a few minutes 

 advances towards the desired fragment, envelops, and incorporates it. 



May I give jou another example also derived from the physiology of 

 plants"? Very shortly after the publication of Engelmann's observa- 

 tions of the attraction of bacteria by oxygen, Pfefler made the remark- 

 able discovery that the movements of the antherozoids of ferns and of 

 mosses are guided by impressions derived from chemical sources, by the 

 allurement exercised upon them by certain chemical substances in solu- 

 tion — in one of the instances mentioned, by sugar, in the other by an 

 organic acid. The method consisted in introducing the substance to be 

 tested, in any required strength, into a minute capillary tube closed at 

 one end, and placing it under the microscope in water inhabited by 

 antherozoids, which thereui)on showed their predilection for tlie sub- 

 stance, or the contrary, by its effect on their movements. 1 n accordciiice 

 with the principle followed in experimental psychology, Pfeffert made 

 it his object to determine, not the relative effects of different doses, but 

 the smallest perceptible increase of dose which the organism was able 

 to detect, with this result — that, just as in measurements of the rela- 

 tion between stimulus and reaction in ourselves we find that the sensa- 



* Lister, "Ou the Plasniodiiiiii of Badhamia ufricxhois, etc. Annals of Botany, 

 No. 5, June, 1888. 



tPfeffer, Untersuch. a. d. hotan. Institute z. Tiibin(jen, vol. i, j)iirt :>, 1884. 



