THE CONDITIONS OF PLANT LIFE 25 
the amount used by the cells in respiration; hence 
when exposed to light the cells give off an excess of 
free oxygen which escapes into the surrounding atmos- 
phere. So soon, however, as the light ceases to act, it 
is found that these cells, like the colorless ones, con- 
sume free oxygen, and the oxidation in the protoplasm 
is accompanied by an evolution of heat, precisely as in 
animals, although, as a rule, it is less energetic. Occa- 
sionally this evolution of heat is quite perceptible and 
can be easily measured. A thermometer thrust into 
a mass of ‘germinating seeds, or into the spathe of one 
of the larger Aroids like the common “calla-lily,” will 
show a rise of several degrees, this evolution of heat 
in the latter case being most energetic while the pollen 
is being shed. 
The gradual evolution of the reproductive parts of 
plants is very instructive, especially when we consider 
the fact that it closely parallels the development of 
these parts in animals, this being especially true of the 
sexual reproductive elements. 
In the lowest forms of life, both plant and animal, 
the entire unicellular organism is at once vegetative 
and reproductive. In these forms, after the cell has 
attained its maximum size, it divides directly into 
two or more parts, each of which becomes at once an 
individual. The power of forming new individuals 
non-sexually persists in many multicellular animals, 
and in most plants. In animals the power of produc- 
ing new individuals by budding or fission is found in 
a considerable number of the lower groups of Metazoa, 
such as the Corals, Sea-anemones, etc., and the renewal 
of lost parts may take place even in Vertebrates, e.g. 
