358 THE EFFECT OF WATER UPON GROWTH [Ch. XII 



by experiment. Upon a glass disc was placed a piece of moist 

 filter-paper so large tliat its edges hung vertically downwards 

 as a flap beyond the margin of the disc. Thalli of various 

 Marchantiaccce were placed in sand at the margin of the disc in 

 such a way that the young growing edge projected half a 

 centimetre beyond. The disc and the object on it were ex- 

 posed to daylight, but slowly rotated in a horizontal plane in 

 order to eliminate phototropic action. The young, positively 

 geotropic, rhizoids which developed beyond the margin of the 

 disc did not grow vertically downwards, but turned towards 

 the flap of moist filter-paper, thus proving that they are posi- 

 tively hydrotropic. The same is doubtless true of the rhizoids 

 of ferns. 



3. Stems. — Very few studies seem to have been made upon 

 the hydrotropism of the stems of seedlings; the most impor- 

 tant are those of Molisch. Several sets of experiments were 

 carried out upon seedlings of flax, pepper-grass (Lepidium 

 sativum), bean, Nicotiana camelina, etc. The method employed 

 was nearly that of Woetmann (see below). Of these plants 

 the hypocotyls of the flax alone showed any hydrotropism ; it 

 may accordingly be concluded that stems are markedly hydro- 

 tropic in but few seedlings. 



4. Pollen-Tubes. — The reactions to moisture of these organs 

 have been studied by Miyoshi ('94). He placed pollen-grains 

 on the stigma of the same species and found that whereas in a 

 dark, moist chamber the pollen-tubes grew in all directions, 

 when dry air was admitted the pollen-tubes turned towards "the 

 centre of the stigma. This turning is best explained as a 

 response to the greater moisture surrounding the mouth of the 

 stigma. It is clearly also an advantageous result, since it tends 

 to direct the pollen-tube to the ovary. 



5. Hyphse of Fungi. — While Sachs ('79) early suggested 

 that the sporangium bearer of Phycomyces nitens is negatively 

 hydrotropic, the first experimental evidence on this point was 

 offered by Woetmann ('81). 



Spores of Phycomyces were sown on bread kept in a moist chamber 

 whose walls were made opaque to prevent phototropism. When, after three 

 or four days, some of the sporangium-bearers had gained a height of one or 

 two centimetres all were bent to one side excepting one which protruded 



