234 PLANT PHYSIOLOGY 



is it attracted by the distinguishing color and form. The 

 direction of locomotion by taste and scent is but the special 

 form of chemotaxis exhibited by higher animals. In the 

 lower animals and in plants, in which the separate senses 

 are not seated in specially differentiated organs, the same 

 sensitiveness of the living protoplasm is exhibited, in a de- 

 gree not always less striking. 



The habit of motile aerobic bacteria of gathering around 

 air-bubbles and at the edges of the coverglass in micro- 

 scopic preparations is of common observation. Correspond- 

 ingly, in preparations made without air-bubbles and cut off 

 from a supply of air by sealing the margins with vaseline, 

 the bacteria become uniformly distributed and presently 

 come to rest. This indicates the dependence of such bac- 

 teria upon oxygen as a source of energy. Their uniform 

 distribution when the supply of oxygen is uniform, and 

 their collecting around the greater quantities when the 

 supply is scattered irregularly, indicate the importance of 

 oxygen in directing as well as maintaining their movements. 

 The chemotactic movement of certain bacteria in relation to 

 oxygen was carefully studied by Engelmann* and made use 

 of by him in determining which of the component rays of 

 sunlight are most efficient in the manufacture of food by the 

 chlorophyll grain (see p. 56). By placing a filamentous 

 alga under the coverglass with the bacteria, and duly illu- 

 minating the preparation, the alga will become photosyn- 

 thetically active and liberate oxygen, the bacteria gather- 

 ing about it. If a short spectrum, instead of white light, be 

 thrown on the alga, the cells exposed to the yellow light 

 will give out most oxygen, as shown by the largest num- 

 ber of bacteria collecting there. 



The chemotactic movements of bacteria with relation to 

 conjugating Spirogyra filaments have already been referred 

 to (p. 232). Bacteria are known to collect on the surface 



'* Engelmann, T. W. Eine neue Methode zur Untersuchung der Sauer- 

 stoffausscheidung pflanzlicher und thierischer Organismen. Archiv. f. d. 

 ges. Physiol., Bd. 25, 1881. L'emission d'oxygene sous Tinfluence de la 

 lumiere par les cellules a chromophylle, demonstres au moyen de la 

 methode bacterienne. Archives Neerland., T. 28. 1894. 



