Sense-Life and Sensibility 
examples, the light rays are focussed on the lower or 
floor-walls of the epidermis, they will produce a differ- 
ence in the illumination of the protoplasm there, and, 
on his theory, this difference is the protoplasm-stimulus 
which results in the leaf taking up a proper light position. 
Each individual cell will move until it is in the right 
light position. There is a difficulty, however, in dis- 
tinguishing between light as producing protoplasm 
movement and light as producing assimilation, but the 
close resemblance between the primitive eyes of animals 
and these epidermis cell eyes of plants inclines one to 
Haberlandt’s theory.’ If one could believe, without 
misgivings, that the older leaves of Heemanthus do really 
twist sideways so as to allow of a better light position 
for the younger ones, then one needs no further proof 
of reason and morality in the plant world, but some 
confirmation is acquired of these observations.® 
Young growing roots, stems, and leaves are very 
sensitive to the earth’s attraction or gravity (that is, they 
are geotropic). In a general way, stems grow up and 
roots downwards, whilst many runners and underground 
stems grow horizontally. One of the most remarkable 
cases is that of the underground rhizome of Solomon’s 
seal which prefers to grow at a definite depth below 
the surface of the soil. If it is planted too deep, it 
will grow in a slanting way upwards till it reaches this 
favoured depth, and will then grow horizontally. If it 
is placed too near the surface it turns downwards till it 
gets to its proper level. 
This remarkable behaviour depends upon two re- 
actions, The upright foliage and flowering shoot, which 
is a bud springing from the rhizome, seems to perceive 
the light stimulus, and as soon as it pierces through 
the soil, the upward growth of the rhizome stops. 
The rhizome itself is influenced by gravity in such 
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