ATMOSPHERE AXJJ CLIMATE, ETC. iy 



exist in spite of the prevalence of all kinds of disease 

 bacteria. It is worthy of note that most of the insects 

 attacking crops only fly and deposit their eggs at night,- 

 while very often the "worms" only feed during the dark 

 hours of the night. Sunlight and heat are the only 

 sources of energy available for carrying on life. The 

 sun's rays supply the energy for plant growth, the energy 

 for the evaporation of water and formation of rains, and 

 supplied the energy which lies hidden in coal. It has 

 been already noticed that when combustion the union 

 of oxygen with some other body takes place, heat is 

 produced ; and the converse is true, that when oxygen is 

 separated from some other body, energy is necessary. It 

 was pointed out that the plant takes carbon dioxide from 

 the air and splits it up, retaining the carbon to build up 

 its body. The source of energy for this separation of 

 oxygen is the sun's rays. Without this source of energy 

 plants could not build up their bodies, and life could not 

 exist. Too much light rather retards plant growth, and 

 consequently at noon many plants "close up" and have a 

 period of rest. Indirectly, light increases transpiration 

 and the rate of the absorption of plant foods. The ravs 

 of light falling on the stem strengthen it, and plant stems 

 which receive little light are weak and liable to be "laid." 

 Thus when wheat or barley is sown very thickly, the 

 lower parts of the stems are unable to obtain sufficient 

 light to strengthen their tissues, and the whole crop often 

 lies down. With forage crops, flax, lettuce, celery, etc., 

 tender vegetation is obtained by sowing thickly or other- 

 wise protecting the sterns from the sun. Weeds arc unable 



