308 A TEXTBOOK OF GENERAL BOTANY 
owing to this fact that land can be fertilized by growing peas or 
beans, especially if the plants are afterward buried in the soil. 
Reproduction of bacteria. Bacteria reproduce by the simple 
division of one cell into two. The divisions frequently follow 
each other in rapid succession, and the length of time from one 
| division to the next may not 
.be more than from twenty 
to thirty minutes. The _ ba- 
cilli and spirilla also produce 
spores. Spores are single-celled 
reproductive structures. The 
spores of bacteria are called 
endospores from the fact that 
they are formed within the 
cell (Fig. 308). The proto- 
plasm of. the cell surrounds 
ty itself with a thick wall within 
bas the original cell wall. The 
\G endospores are very resistant 
to external conditions, such 
as dryness or extreme tem- 
peratures. ‘Their importance 
j seems to lie in the fact that 
they enable the bacteria to 
survive under adverse cir- 
cumstances. Under favorable 
conditions the covering of the 
spore bursts and the protoplasm resumes its ordinary activities. 
Sources of energy, Green plants obtain energy from sunlight 
and store it in the products of photosynthesis. When energy 
is required for the life activities of these plants, this stored 
energy is released by respiration, which is the oxidation of 
organic compounds that had their origin in the process of photo- 
synthesis. Animals also obtain energy for their life activities 
by the release, through respiration, of energy stored by plants in 
the products of photosynthesis. The energy used by both plants 

Fig. 310. Nodules on root of sensitive 
plant (Mimosa pudica) due to nitrogen- 
fixing bacteria. (x 1) 
