LECTURE XVIII. 



prove that the stimulus in question is due to the dampness of 

 the moist surface, but special experiments have been made 

 which supply the necessary evidence. Knight and Johnson 

 found, for example, in the course of their experiments, that 

 when the air into which the roots of the seedlings penetrated, 

 after escaping through the holes in the vessel, was saturated 

 with moisture, the roots continued to grow vertically down- 

 wards. The curvature is, then, the response of the root to the 

 stimulating influence of the moist surface, and it is to this that 

 Darwin has applied the term Hydrotropism. 



We see that a root behaves to the stimulating action of a 

 moist surface much in the same way as it does to the action 

 of gravity ; so we may say that a root is positively hydro- 

 tropic just as we say that it is positively geotropic. We may 

 also go a step further and enquire if there are such things as 

 negatively hydrotropic organs ; we might expect, for instance, 

 that since roots are positively hydrotropic, stems would be 

 negatively hydrotropic. It appears, from the researches of 

 Molisch, that stems, with the possible exception of the hypo- 

 cotyl of Linum usitatissimum which exhibited some signs of 

 negative hydrotropism, are not hydrotropic at all. Wortmann 

 has, however, observed that the sporangiferous hyphse of 

 Phycomyces, are distinctly negatively hydrotropic, for they 

 curve away from any moist surface which may be brought 

 near to them. Wortmann's observations have been confirmed 

 by Molisch, who has further found that the subaerial hyphae 

 of Mucor stolonifer and the stipes of Coprinus, are also nega- 

 tively hydrotropic, whereas the rhizoids of Marchantia and 

 other Liverworts are positively hydrotropic. 



Molisch points out with regard to the hydrotropism of 

 roots, that, as might be expected, they curve out of the vertical 

 towards a moist surface the more readily the less they are 

 acted upon by gravity. For instance, we have learned that 

 lateral roots are less geotropically irritable than primary roots, 

 and Molisch has observed that the former curve hydrotropically 

 more readily than the latter. 



These are the principal facts which are at present known 

 concerning hydrotropism. We see, at once, that we have to 



