LECTURE XX. 



was supported by Baranetzky, who made the further re- 

 markable observation that by a lowering of the temperature 

 and exposure to light the negative geotropism is converted 

 into positive. Strasburger, Pfeffer, and Stahl, however, 

 failed to detect any indication of geotropic irritability in the 

 plasmodia. Strasburger and Stahl shew, on the contrary, 

 that the apparently geotropic phenomena are due to alto- 

 gether different causes ; either to the direction of the 

 current of water by which the plasmodia were moistened 

 during the experiment, or to differences of moisture in the 

 neighbourhood. Strasburger and Stahl have conclusively 

 proved that the plasmodia are negatively rheotropic, that 

 is, when a stream of water is allowed to trickle over them, 

 they move in a direction opposite to that of the stream ; 

 a result which has also been obtained by Jonsson. Stahl 

 has further ascertained that the plasmodia are positively 

 hydrotropic during the greater part of their existence, that 

 is, that they move towards the most moist spot in their 

 neighbourhood. When, however, they are about to form 

 sporangia, they become negatively hydrotropic, and move 

 away from moisture. It is on this account, as Stahl points 

 out, and not on account of negative geotropism, that 

 plasmodia which are about to produce spores come to the 

 surface of the substratum or even climb up on various 

 objects such as plants, etc. Further it has been ascertained 

 that plasmodia are thermotropic. Stahl observed that when 

 a strip of blotting paper, with a plasmodium on it, was 

 placed on the adjacent margins of two vessels containing 

 water, the one at f C, the other at 30 C, the plasmodium 

 moved towards the vessel containing water at the higher 

 temperature. This observation has been confirmed by 

 Wortmann, who has further ascertained that at relatively 

 high temperatures the plasmodium is negatively thermo- 

 tropic. On repeating Stahl's experiment, the water in 

 the cooler vessel being at I5-2OC., and in the warmer 

 at 40 C., he found that the plasmodium moved away from 

 the water on both sides. He determined the maximum 

 temperature for positive thermotropism to be 36 C. 



