§ 2] HYDROTROPISM 359' 



through a small hole in the glass disc. Parallel to this hypha and close to 

 it was placed a piece of soaked card. After 4 to 6 hours the hypha turned 

 from the damp card; but when the card was dry no such turning occurred. 



The extraordinary sensitiveness of the sporangium bearing- 

 filaments of Phycomyces has been shown by the experiments of 

 Eeraea ('93). He found that these organs turned toward 

 rusting iron, cliina-clay, agate (but not rock crystal), and sul- 

 phuric acid. He explained this result on the ground that 

 these substances absorb the moisture in their vicinity. Con- 

 sequently the hydrotropic filaments turn towards this rela- 

 tively dryer region. So sensitive, indeed, is this plant that it 

 may be used to detect a very slight difference in the hygro- 

 scopic properties of chemically related substances. 



Not only do the hyphte of Phycomyces turn from moisture, 

 but, as MOLISCH ('83) showed, those of Mucor stolonifer and 

 the relatively great trunk of the toadstool Coprinus velaris 

 respond in the same way. The spores are thereby carried 

 away from the moist situation. 



In conclusion a word may be said concerning the cause of 

 hydrotropism. It is probable that two diverse phenomena are 

 confused under the term. One of these is seen when a multi- 

 cellular organ like the root of phanerogams is unequally 

 moistened on opposite sides ; the moister side will lose water 

 less quickly than the other, or it may actually imbibe some. 

 Its cells will accordingly become more turgescent and the 

 whole moister side more convex. This result is due to a 

 relatively direct, almost mechanical, cause ; it simulates nega- 

 tive hydrotropism, but it is so different in kind from the true 

 phenomenon that it may be called false hydrotropism. 



In the second class of cases we see multicellular organs, such 

 as roots, becoming concave towards the slightly moister region, 

 or unicellular organs, such as rhizoids, pollen-tubes and hyphse 

 — organs which cannot be supposed to become unequally tur- 

 gescent on the two sides — exhibiting a -|- or — turning. 

 These cases cannot be explained on direct mechanical grounds ; 

 they are responses to stimuli, and, as such, examples of true 

 hydrotropism. 



These two kinds of hydrotropism may occur in the same 

 organ under different conditions and thus cause turnings in 



