ADDRESS. 25 



ganisms to be observed are not coloured, but bacteria of that common 

 sort which twenty years ago we used to call Bacterium termo, and which 

 is recognised as the ordinary determining cause of putrefaction. These 

 organisms do not care for light, but are great oxygen-lovers. Conse- 

 quently, if you illuminate with your spectrum a filament of a confervoid 

 alga, placed in water containing bacteria, the assimilation of carbon and 

 consequent disengagement of oxygen are most active in the part of the 

 filament which receives the red rays (b to c). To this part, therefore, 

 where there is a dark band of absorption, 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 which the 

 source of attraction is food. 



The Plasmodia of the 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 living 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, tlie dogfish is attracted 

 by food which it cannot see, so the plasmodium of Badhamia 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 centre of activity 

 streams of living material converge. Soon the afHux leads to an out- 

 growth of the Plasmodium, which in a few minutes advances towards the 

 desired fragment, envelopes, and incorporates it. 



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

 plants ? Very shortly after the publication of Engelmann's observations 

 of the attraction of bacteria by oxygen, PfeSer made the remarkable 

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

 are guided by impressions derived from chemical sources, by the allure- 

 ment exercised upon them by certain chemical substances in solution — 

 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 thereupon showed their predilection for the substance, or the 

 contrary, by its effect on their movements. In accordance with the 



' Lister, ' On the Plasmodium of Badhamia utricularis, &c.,' Annals of Botany, 

 No. 5, June 1888. 



