sect, n PHYSIOLOGY 243 



serviceable to the protoplasmic organisms through the irritability of 

 the protoplasm. Gravity and light, certain substances in solution, 

 and mechanical hindrances are the principal influences which regulate 

 the movements of free -swimming protoplasmic, bodies and cells. 

 The direction of the movements of the swarm-spores of Algae are 

 chiefly determined by the light. So long as they remain in darkness 

 they move through the water in all directions ; but as soon as they are 

 illuminated from one side only, a definite direction in their movements 

 is perceptible. They move either straight towards the light or turn 

 directly away from its source. Their retrogressive movements from 

 the light occur either in case of too intense illumination, or at a certain 

 age, or through some unknown disturbing irritation. The advantage 

 of such HELIOTACTIC movements (phototactic) is at once apparent when 

 the part taken by the swarm-spore in the life of an Alga is considered. 

 In order to provide for the future nutrition of the stationary Alga 

 into which it afterwards develops, it must seek the light. If a point 

 with suitable (that is, not too intense and not too weak) illumination 

 be attained, then the swarm-spore must attach itself by the end which 

 carries the cilia : to do this it must turn itself from the light towards a 

 dark object. On the other hand, as the swarm-spores do not come to 

 rest at all in absolute darkness, but swim continuously until thoroughly 

 exhausted, the possibility of their attaching themselves in a spot devoid 

 of light is excluded, and where the new plant could not assimilate. 



The swarm-spores of water Fungi and motile Bacteria, according to 

 Peeffer's investigations, are chiefly influenced in their movements by 

 the unequal distribution of dissolved, solid, or gaseous matter (oxygen) 

 in their environment. According to their momentary requirements and 

 their sensitiveness to stimuli, they move either towards or away from 

 the points of highest concentration. 



As the result of similar chemotacttc movements spermatozoids 

 approach the female sexual organs. Pfeffer has demonstrated that the 

 spermatozoids of Ferns are enticed into the long necks of the archegonia 

 by means of malic acid : while the archegonia of the Mosses attract the 

 spermatozoids by a solution of cane-sugar. In such cases an extremely 

 small quantity of dissolved substance is often a sufficient stimulus to call 

 forth active chemotactic movements ; a O'OOl per cent solution of malic 

 acid suffices for the attraction of Fern spermatozoids. The movements 

 of amoeba? and plasmodia are similarly induced by external influences. 

 These naked protoplasts live not only in water (amoebae), but also 

 in moist substrata (plasmodia, amcebas), and seem to possess the 

 power of seeking out situations with more moisture, or of avoiding 

 them (before the formation of spores) : their movements are also 

 influenced by the direction of currents in the water (rheotaxis). In 

 cases where cells enclosed by cell walls (Sphaerella pluvicdis) swim freely 

 about by means of cilia, the cilia spring from the protoplasm and 

 pierce the cell walls. 



