24 EVOLUTION OF PLANTS 
moulds, Fig. 4), when exposed to light, the green 
swarm-spores of alge almost always being very quickly 
attracted to the light. 
Geotropism, or the movements induced by gravity, 
are either negative or positive. In the higher plants, 
the aerial parts, especially the stems, are usually nega- 
tively geotropic, 7.e. grow upward, the roots positively 
geotropic. 
The marked movements of the sensitive plant (Mi- 
mosa), as well as the similar movements especially of the 
floral parts of many plants, and the so-called “sleep move- 
ments” of such leaves as the locust, clover, and many 
others, are connected with changes in the turgor of the 
cells of special parts of the motile organs. These 
movements are undoubtedly, like the movements due 
to simple heliotropism, intimately associated with the 
sensitiveness of the protoplasm, and are induced by a 
variety of stimuli, such as shocks, light, and electricity. 
The absence of locomotion in the higher plants is in 
large measure due to the investment of the cells with a 
firm membrane, and it is only when the protoplasm 
escapes from the cell, as in the case of swarm-spores 
and spermatozoids, that the primitive power of locomo- 
tion is regained, and recalls the possession of this 
animal character by the ancestors of all the higher 
plants. 
It is a popular fallacy that plants and animals supple- 
ment each other in their method of respiration. It is 
not necessary to remind the botanist that this mistake 
is based upon a confusion of terms. During the process 
of carbon assimilation in the green cells, there is a large 
amount of free oxygen liberated — much in excess of 
