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. 



