2] THIGMOTROPISM 381 



sustained by later work. Thus DETI^EFSEN ('82) found that- 

 bean radicles plunged right through tinfoil 0.0074 mm. in 

 thickness, and WIESNER ('84) and SPAULDING ('94) have shown 

 that the turning from the gummed paper is the result of the 

 injurious action of the gum. Indeed, it is difficult to see how a 

 root, which must force its way through the earth, should turn 

 from a solid surface. Experience shows rather that it turns 

 towards it and tends to grow along it or through it. 



4. Cryptogams. Among Fungi a thigmotropic response has 

 been observed and studied in the mold Phycomyces nitens by 

 ERRARA ('84) and WORTMANN ('87). They find that rub- 

 bing the sporangiferous hypha lightly with a bristle or glass 

 thread for from three to six minutes will produce a response 

 more powerful even than that to light. As with tendrils, 7% 

 to 14% gelatine, almond oil, and water drops provoke no bend- 

 ing ; but, on the other hand, even the mutual rubbing of two 

 adjacent sporangiferous hyphse may incite a response. The 

 response will, however, appear only when the growing region 

 of the hypha is irritated ; if the hypha is fully grown, no thig- 

 motropism will take place. The sharpest part of the bend will 

 occur at the region of greatest growth, not necessarily at the 

 point of contact. However, WORTMANN finds that the turn- 

 ing commences at the point of contact, but becomes more and 

 more pronounced as the bend approaches the most rapidly 

 growing part of the hypha. The response to a brief contact 

 is only temporary. Thus when a sporangiferous hypha 2.8 cm. 

 long was touched for one minute with a glass thread a bending 

 began to appear after 15 minutes and disappeared 10 minutes 

 later. Thus Phycomyces follows closely the laws of response 

 of tendrils. 



The rhizoids of the hepatic Marchantia and its allies have 

 been found by MOLISCH ('84, p. 933) to behave in a similar 

 way to the foregoing. When the hepatics are placed on damp 

 filter-paper, over a watch-glass, in a moist chamber and in the 

 light, one finds after 48 hours that rhizoids have penetrated 

 through the filter-paper. Since the paper reveals, even to the 

 microscope, no pores, and since grains of corn starch, of from 2 

 to 24 microns diameter, will not pass through it, we must con- 

 clude that the root hairs of 10 to 25 microns diameter, although 



