GROWTH AND MOVEMENT 475 



Hydrotropism. Another special form of chemotropism, which has 

 been named hydro tropism, designates the sensitiveness of roots, the 

 hyphae of some fungi, the rhizoids of liverworts, etc., exhibited by turn- 

 ing toward or away from the source of diffusing water vapor, or capil- 

 lary water in. soils. When seedlings are grown in an atmosphere less 

 than saturated with water vapor, so that the roots, as they grow, pass 

 further and further away from a wet surface, 1 it will be found that they 

 deviate presently from the perpendicular, inclining toward the wet 

 surface; soon again they turn downwards, but once more return to the 

 moisture, and this may be repeated many times. Plainly the roots are 

 subject here to two stimuli acting nearly at right angles, gravity and the 

 diffusing vapor. First the one dominates and then the other. Were it 

 not for the long reaction times the root might be expected to take an inter- 

 mediate direction, the resultant of the effects of the two stimuli; but as 

 in the case of geotropism alone (see p. 460), the after-effects carry the root 

 tip past the position of equilibrium, whereupon the other stimulus gives 

 it such strong and long excitation that its after-effects carry the root 

 again past the equilibrium point; then the gravity stimulus comes upon 

 it again; and so it weaves back and forth. 



The vegetative hyphae of the mold fungi may show positive hydro- 

 tropism and their sporangiophores negative hydro tropism. It can 

 easily be shown that the rhizoids of Marchantia, which normally grow 

 straight downward, will deviate toward a moist surface in the same way 

 as roots; only the moisture stimulus is dominant over gravity. Roots 

 in the soils also grow towards the moister regions, and especially do they 

 tend toward tile drains, into which they may penetrate, often branching 

 profusely enough to plug up the drain completely. Part of this direc- 

 tive effect may be due, and probably most of the branching is due, 2 to 

 chemical stimulation by the solutes. 



(6) PHOTOTROPISM 



Stimulus. Of all the external conditions that act upon plants, light 

 is one of the most variable, for from time to time it differs in direction, 

 in intensity, and in quality. Quite apart from its fundamental relation 

 to all life in furnishing the energy for food making, are its effects as a 

 stimulus. Whereas the most effective quality of light for food making 



1 As by planting them in coarse sawdust held in place on the under surface of an inclined 

 board by bobbinet. 



2 In 1 which case this is a morphogenic effect. See p. 435. 



