380 EFFECT OF MOLAR AGENTS UPON GROWTH [Cn. XIV 



straightening again. These periods are represented graphically 

 in Fig. 102. The latent period is of variable length in differ- 

 ent species. Perhaps the shortest period is that of Cyclan- 

 thera, which MULLER ('86, p. 103) found to vary in the most 

 sensitive condition of the plant from 5 to 9 seconds. DARWIN 

 ('82, p. 172) found a latent period in Passiflora tendrils of 25 

 to 30 seconds, while in other cases (Dicentra smilax, Ampe- 

 lopsis) this period may be 30 to 90 minutes or more in length. 

 The entire time required for the plant to straighten completely 

 again is, according to MULLER ('86, p. 104), for Cyclanthera 

 about 20 minutes. These times depend, however, to a large 

 extent upon various external agents. Thus a quick response 

 is favored by a temperature between 17 and 33 C:, sunlight 

 rather than shade, considerable humidity, and small size of ten- 

 dril. Thus this growth response resembles, in its dependence 

 upon various conditions, other responses, e.g. those of muscle, 

 to stimuli. 



3. Roots.- -Thigmotropism in roots was apparently first 

 investigated by SACHS ('73, pp. 437-439). He fastened ger- 

 minating seeds of various species in a moist chamber so that 

 their radicles, of about 10 to 30 mm. length, were horizontal. 

 A pin was now placed against the root near its tip so as to 

 exert considerable pressure. Usually within eight or ten 

 hours a turning of the root in the growing region occurred so 

 that it became concave towards the pin and finally made a 

 complete loop about it, or, if the needle was vertical, descended 

 along it in a spiral a result of the double action of thigmo- 

 tropism and geotropism. This response was, however, rather 

 inconstant and appeared not to be very powerful. 



The sense of the thigmotropic response was, in the foregoing 

 example, positive ; that is, the turning was towards the solid 

 body. DARWIN ('81, p. 131) believed that he had evidence 

 that the response is sometimes negative. Thus he says that 

 the radicle of a germinating bean, coming in contact with a 

 sheet of extremely thin tinfoil (0.003 to 0.02 mm. in thick- 

 ness), was in one experiment deflected without making a groove 

 upon it ; also, when a piece of paper was gummed to the root 

 near the tip, the root usually turned so that it became convex 

 to the gummed paper. But DARWIN'S conclusion has not been 



