THE STEM 109 
but is caused by differences inherent in the organs themselves. 
The response to gravity is brought about by different rates of 
growth on the upper and lower sides of the organs concerned, 
the side which becomes convex growing more rapidly than the 
opposite side. Geotropism is largely responsible for the position 
of upright stems. Branches do 
not show the action of geotro- 
pism to the same extent as 
does the main axis. 
Phototropism. Stems tend to : yi ! Thi, 
grow toward the light, while ie ie Ly 
most aérial roots grow away Ly BU 
from the ight (Fig. 106). The 
tendency of plant organs to 
orient themselves with refer- 
ence to light is called photo- 

i), 
ity 
Vif] 




Fig.107. A radish seedling grown on 
tropism. An organ that grows 
toward the light exhibits posz- 
tive phototropism, while an or- 
gan that grows away from the 
light shows negative phototro- 
the side of a piece of blotting paper 
held vertically in a darkened box 
with light entering on the left side 
The position of the shoot is due to a 
combination of positive phototropism 

and negative geotropism; that of the 
root, to negative phototropism and 
positive geotropism. (x 1) 
pism. Nearly all roots that 
normally grow in the ground 
show little or no _ phototro- 
-pism; but there are some exceptions, as in the case of the 
radish, where the roots are negatively phototropic (Fig. 107). 
The effect of phototropism can often be clearly seen in up- 
right plants growing very near a house. In such cases there is 
a tendency for the branches to grow away from the house, or, in 
other words, toward the source of greatest illumination. There 
is also a tendency for the main stem to lean away from the 
house, or toward the light. In trees that bend toward the light 
the position of the main axis is very clearly the result of the 
combined action of geotropism and phototropism. ‘The gen- 
eral upright position of the stem is due to geotropism, while 
the leaning position is the result of phototropism. The effects 
