434 PHYSIOLOGY OF GROWTH. 



fully, and it is best to convey the tan from the tannery to the 

 laboratory in a box, without fingering it much. We now conduct 

 the following experiment : 



A small strip of Swedish filter-paper is moistened and dipped 

 at one end into a beaker half full of water. The other end of it 

 hangs freely downwards over the edge of the beaker, and during 

 the experiment is spread out on the plasmodium-bearing tan. If 

 we now place the whole preparation in darkness, in a room whose 

 temperature is 25 to 30 C., plasmodia soon leave the tan, and 

 creep further and further up the strip of filter-paper. This 

 migration of the plasmodia, on a substratum thoroughly saturated 

 with water, is not due to hydrotropism, but is induced by 

 currents of water. The plasmodia are therefore rheotropic, and 

 in fact they always travel against the current of water. 



The plasmodia, however, react also to variations in the distri- 

 bution of water in the substratum ; they are not only rheotropic, 

 but also hydrotropic. To prove this we convey plasmodia, which 

 have collected on blotting-paper under the influence of a current 

 of water, to the middle of a sheet of glass covered with several 

 folds of moist filter paper. The plasmodia, when placed in a 

 dark chamber in which the air is saturated with moisture, spread 

 out uniformly on the horizontal and water-saturated substratum. 

 If the research material is now placed in a dark chamber which 

 however is dry, and we fix at a short distance above the 

 plasmodia an object-glass smeared with dilute gelatin, an inter- 

 esting appearance is soon (often after a few hours) to be observed. 

 The blotting-paper gradually dries, and the plasmodia withdraw 

 from the drying portions of the substratum, but accumulate 

 under the moisture-distributing object-glass. The plasmodia 

 thus exhibit positive hydrotropism. It must be remarked that 

 our plasmodia, during the greater part of their development, 

 react as above to variations in the distribution of moisture. 

 Plasmodia near the time of sporangium development are, on the 

 contrary, negatively hydrotropic. 



The plasmodia of the Myxomycetes are not at all geotropic. 

 This can be demonstrated by placing strips of blotting-paper 

 with plasmodia against a vertical, moist substratum (e.y. paper 

 saturated with water, and lying on a sheet of glass) in a dark 

 chamber saturated with moisture. Under these circumstances 

 the plasmodia spread out uniformly in all directions on the sub- 

 stratum. 



