THE CONDITIONS OF PLANT LIFE 23 
which exists between the lowest members of the two 
great organic kingdoms. This power of free locomotion 
is also found in some of the reproductive cells of most 
plants except the highest ones, and even in a few of 
the seed plants these have recently been discovered, 
the male reproductive cells having the form of ciliated 
spermatozoids which are actively motile. This power 
of spontaneous locomotion is finally lost, and in nearly 
all the seed plants is completely absent, although even 
here the plant exhibits more or less marked movements 
of various kinds. Some of these movements are appar- 
ently spontaneous, such as the revolution of the apex 
of the growing stem and root, and of tendrils, but oth- 
ers are influenced directly by external agencies, light, 
moisture, contact, and gravity. The spontaneous move- 
ments of the growing apex of many plants is called 
nutation, and is apparently quite independent of ex- 
ternal agencies. The effect of light in plants is well 
known. ‘The response of actively growing plants to 
this stimulus is often very rapid, although the exact 
mechanism of these movements is not entirely clear. 
Occasionally plants are negatively heliotropic, z.e. they 
grow away from the light, as is seen in the commen ivy. 
Light is, in most cases, necessary for the formation of 
chlorophyll, as well as for the performance of photo- 
synthesis in those cells which contain the chlorophyll. 
_Doubtless the cause of the movements of plants is 
largely due to the direct effect of light upon the sen- 
sitive protoplasm of the cells of the motile parts. 
This is indicated by the activity of naked swarm- 
spores and plasmodia (masses of naked protoplasm 
found in certain low organisms, such as the Slime- 
