GROWTH AND MOVEMENT 475 
Hydrotropism. Another special form of chemotropism, which has 
been named hydrotropism, 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 hydrotropism. 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 ^o 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 which case this is a morphogenic effect. See p. 435. 
