HYDROTROP1SM 485 



on roots although in other respects the two series of phenomena closely resemble 

 each other. As in the case of aerotropism we had to take account of perception 

 of unequal distribution of a gas, so in hydrotropism we have to deal with unequal 

 distribution of water vapour. SACHS (1872) demonstrated hydrotropism in the 

 root in a very simple way (Fig. 152). He covered a shallow zinc cylinder on 

 one side with a large-meshed netting, filled the cylinder with wet sawdust, and 

 suspended the whole apparatus so that the perforated under side of the cylinder 

 formed an angle of 3o-45 with the horizontal. Peas were then planted in 

 the sawdust, and the roots on germination soon exhibited positive geotropism, 

 growing out of the sawdust through the meshes into the air. ' If the air outside 

 be completely saturated or nearly so, the roots grow straight out into the air ; 

 if the air be not saturated but only moderately damp, the roots bend sideways 

 and curve over until they again reach the under side of the sawdust. Not 

 infrequently they grow backwards closely adpressed to the oblique surface and 

 sometimes the root apex re-enters the moist sawdust through the meshes, at 

 once curving out again geotropically and repeating the performance several 

 times thus lacing itself through the meshes.' 



MOLISCH (1883) has shown that the stimulus in positive hydrotropic curva- 

 ture is perceived by the root apex. He surrounded the root tightly with moist 

 blotting paper so that only about i mm. of the apex was exposed. When a 

 psychrometric difference was established, positive hydrotropic curvature ensued, 

 just as when the growing region was subjected to dissimilar amounts of water 

 vapour on either side. PFEFFER (1894) has more recently extended this research 

 by showing that the stimulus is perceived only by the root apex. If the apex be 

 uniformly wet on all sides hydrotropic curvature never occurs in the growing zone. 

 Further experiments have yet to be undertaken to determine whether special 

 emphasis is to be laid on this difference between aerotropism and hydrotropism, 

 viz. the great senitiveness of the apex in the latter case and the absence of that 

 sensitivity in the former. 



We will only note further the fact that hydrotropism is by no means confined 

 to the root. Positive hydrotropism is exhibited also by the rhizoids of the 

 Marchantiaceae and negative hydrotropism by certain, but by no means all, 

 plumules, e.g. Linum (MOLISCH, 1883), potato (VocHTiNG, 1902), (compare 

 SINGER, 1903, Ber. d. bot. Gesell. 21, 175) ; negative hydrotropism is on the 

 other hand of very general occurrence in Fungi (Mucor, Phycomyces, Coprinus). 

 The sporangiophores of Phycomyces are remarkably sensitive to water (WORT- 

 MANN, 1881) and lead to quite special phenomena in this case. If Phycomyces 

 be grown on bread, it may be noticed that the sporangiophores, though geotropic, 

 do not, in darkness, stand perfectly erect ; they form rather a tuft of diverging 

 filaments. Each sporangiophore, owing to transpiration, renders the air in its 

 immediate vicinity damp, and hence the neighbouring sporangiophores curve 

 away from it, and since the same applies to all of them, the result is this 

 outward divergence of the sporangiophores, the one from the other. The 

 marked attraction of Phycomyces by certain insoluble bodies such as iron, which 

 ELFVING (1890) attributed to physiological stimulation at a distance, is due to 

 hydrotropism (ERRERA, 1892 ; STEYER, 1901). Iron is hygroscopic and hence 

 the air in the neighbourhood of that metal is somewhat drier than before ; 

 Phycomyces bends therefore negatively hydro tropically towards the drier region. 

 Obviously it reacts to very minute psychrometric differences, but exact experi- 

 mental data on the subject are not as yet forthcoming. 



The behaviour of Phycomyces when placed on a klinostat is also quite 

 peculiar ; the sporangiophores stand at right angles to the surface of the rotat- 

 ing medium, just as if there were a radially acting force emanating from it. In 

 reality the position which Phycomyces takes up in relation to the substratum 

 can be explained by hydrotropism only, since it alters when the air is saturated. 

 At the same time it must be noted that an ordinary moist chamber is not ab- 



