450 SECOND PA R T. —MYCF.TOZOA . 



soon as they arc offered to them, here too without regard to the direction in space in 

 which the movement has to he made. II tin- plasmodium of Fuligo which usually 

 lives in tan is spread out on the moist vertical wall of a glass, it remains in this 

 position, other things being the same, as long as the surface of the glass is covered 

 with a film of pure water. If an infusion of tan is added to the water, in such a 

 way that the plasmodium is touched by it at one spot only, it begins to move rapidly 

 towards this spot and gradually puts out numerous branches which dip into the 

 infusion. A small piece of tan placed close to a plasmodium under similar conditions 

 is quickly seized by a number of freshly protruded branches. The similarity in the 

 effect of the fluid containing the infusion of tan and the solid piece of tan shows that 

 it can only be due to chemical constituents of the tan ; what these are has not been 

 precisely ascertained. 



If a plasmodium comes into contact on one side with other bodies dissolved 

 in water, the opposite effect is produced, namely repulsion of the plasmodia. Even a 

 solution containing | or { per cent, of grape sugar produced this effect at first in 

 Stahl's experiments, but the plasmodium by degrees became accustomed to it and 

 behaved to it as to the infusion of tan. A sudden change in the concentration of the 

 saccharine solution, either by increasing it to a certain amount (2 per cent.) or 

 diminishing it, gives rise to similar phenomena to those first described. Stahl 

 observed the same repulsions in experiments with saline solutions. 



If oxygen is excluded on one side, a movement takes place, as might have been 

 expected beforehand, towards the side where oxygen is admitted. 



Thermotropism. If the substratum, other conditions being the same, is 

 unequally warmed on different sides, the plasmodium moves, at least within the limits 

 of temperature observed in Stahl's experiments (+7°to 30°C), towards the side which 

 is most highly warmed. 



Most of the phenomena observed in spontaneously vegetating plasmodia, 

 especially their creeping hither and thither and in and out, according to the time of 

 the year and the state of the weather, on the substratum of vegetable remains, such as 

 leaves, tan and the like, may be explained very simply from the experimental results 

 here recorded. 



A further fact established by Stahl must be added here in explanation of another 

 and very remarkable phenomenon, namely that in the plasmodia of Fuligo and some 

 spe< ies of Physarum, in which the point could be examined, the reaction against 

 locally unequal distribution of water in the environment changes with the age. The 

 plasmodia are positively hydrotropic, that is wander from the dry to the moister spots, 

 other things being equal, during the vegetative stage, but become negatively, 

 hydrotropic near the moment of formation of sporangia, that is they move from the 

 moister to the drier spots. 



This movement also takes place without regard to the mere direction in space, 

 and so may he upwards or downwards &c, and it explains the general fact that almost 

 all plasmodia, as soon as they are ripe for forming sporangia, move to comparatively 

 very dry spots on the surface of the moist substratum, often travelling a considerable 

 distance, before being transformed into sporangia; it explains also, according to 

 Stahl's observations, the elevation of the commencing sporangium in a direction at 

 right angles to the comparatively moist substratum. 



