150 UNIVERSITY OF MISSOURI BULLETIN 



feet spheres. Closer observation shows that each tiny ball 

 consists of an outer, transparent wall containing, within, a 

 colorless, granular substance, and one or more prominent 

 green bodies which give the green color to the mass on the 

 tree-trunk. 



If we place one of these bodies in water containing cer- 

 tain inorganic chemical compounds, and keep it in diffused 

 light it will absorb materials from without, and transform 

 them into substance like itself. Hence, it increases in size, 

 or grows. All of this activity requires energy, part of which 

 is obtained by transforming the energy of sunlight, and part 

 by taking in external oxygen, and oxidizing its own sub- 

 stance. Thus it is not only being continually built up, but 

 at the same time continuously torn down. This fact stamps 

 the body at once as a living thing, for the property of under- 

 going continual destruction and automatic reconstruction is 

 a fundamental distinction between that which is alive and 

 that which is not. The ability to exist and grow when sup- 

 plied with only inorganic chemical compounds reveals the 

 body to us as a plant, for, with a few exceptions among the 

 lower forms, this ability is lacking in animals. From its 

 structure we realize that this plant is composed of a unit 

 mass of matter in the living state (protoplasm), surrounded 

 by a wall, thus constituting what is known as a plant-cell. 



At first the surface area of our tiny plant is amply suffi- 

 cient to permit the entrance of enough raw materials to 

 nourish it, but, being a sphere, the plant is bound by a geo- 

 metrical relationship of all spherical bodies — namely, that 

 its volume increases with the cube of its diameter, but its 

 surface only as the square of the diameter. Thus a stage 

 is soon reached when the surface area becomes too small 

 to permit the entrance of enough food materials to nourish 

 the mass, and the plant must do one of two things — increase 

 its surface area, or die. If external conditions are favor- 

 able, the former will occur, for natural death has never been 



