66 MOISTURE AND PROTOPLASM [Ch. II 



§ 4. The Determination of the Direction of Move- 

 ment BY Moisture, — Hydrotaxis 



This phenomenon has been described by Stahl ('84) in 

 Jiithalium. When this Myxomycete is placed in the dark upon 

 a glass plate covered with several layers of moistened filter 

 paper, it expands uniformly over the homogeneously moistened 

 substratum. If, now, the plate be placed in a drying chamber, 

 the paper dries slowly, and one can see that the mass of the 

 Plasmodium draws towards those places which remain longest 

 damp. If a dilute gelatine jelly is smeared upon a glass slide 

 supported in a horizontal position about 2 mm. above the 

 Plasmodium, still in the dark chamber, the plasraodium sends 

 up branches, some of which may touch the gelatine and spread 

 out over it. If the water dries still further, the entire Myxomy- 

 cete may become transferred to the slide above. If, now, the 

 paper be moistened again, the plasmodium sends branches down 

 to it. Stahl's explanation is that of the old mechanical 

 school. He says, the peripheral protoplasmic layer lying next 

 the dryer region is poorer in water; while that next the 

 damper part of the substratum contains much water. If it be 

 assumed that the internal streaming tends to occur uniformly 

 towards all points of the periphery, it is clear that the dryer, 

 more consistent part will offer greater resistance than the more 

 fluid part, and in this part, therefore, branches will tend to 

 arise. In correspondence with the interpretations which we 

 have hitherto placed upon similar phenomena I prefer to call 

 this a case of response to the stimulus of excessive moisture — 

 in any case it may be designated positive hydrotaxis. 



When, however, the plasmodium of ^Ethalium is in the fruit- 

 ing stage, it retreats from the moister part of the substratum, 

 and other Myxomycetes in the fruiting stage show the same 

 negatively hydrotactic tendency. Thus the same agent, water, 

 stimulates the same organism, at different stages, to reverse 

 movements. 



I will now summarize the conclusions concerning the effect 

 of water upon protoplasm. Water constitutes by far the larger 

 part of protoplasm and of all active organisms. Metabolism is 



