56 INTRODUCTION TO GENERAL PHYSIOLOGY 



We see, however, how dependent we are on the sun for our 

 continued existence, and more definitely how the "struggle for 

 existence " is one for the possession of the free energy of the sun's 

 rays. 



The assimilation of carbon dioxide and the production of 

 oxygen is the great function of the enormous area of green leaves 

 that is to be seen on the earth. Since it is necessary for them to 

 receive as much light as possible, we see why they are in the form 

 of thin sheets, and why they spread themselves out in such a way 

 as to receive the maximum amount of light. Sometimes in the 

 tropics the sun's light is destructively brilliant, and the chloroplasts 

 take such positions as not to be subject to its full intensity. 



The Cycle of Nitrogen 



As already stated, waste nitrogen leaves the animal body 

 chiefly in the form of urea, a small amount as more complex 

 compounds. When death takes place, the proteins of the tissues 

 are broken up by the agency of those minute vegetable organisms 

 called "bacteria." In the case of plants, although their protoplasm 

 probably gives off some simple substance, such as urea, in the 

 course of its chemical changes this is very small, and the nitrogen 

 of the plant structures finally passes to the soil in the same way as 

 that of the animal body does. 



What, then, are bacteria? They are microscopic plant organ- 

 isms, devoid of chlorophyll, and exist in great variety of forms 

 and properties (E., p. 186). They are present all around us, to 

 some extent blown about in the air, but chiefly on the surfaces of 

 all kinds of materials and in the waters and soil. One of the most 

 striking things about them is that, although the number of distinct 

 shapes assumed is not great, the chemical activities they perform 

 are of an enormous variety. The chief forms are : small spheres 

 (micrococci), sometimes attached together in chains, bacteria (short 

 rods with rounded ends), bacilli (longer rods with flat ends), and 

 twisted rods of various lengths and closeness of coils (spirilla and 

 vibrios). Some of these in certain stages move about by the 

 agency of threads which are contractile (flagella). Others are 

 frequently found in a resting state as spores, surrounded by a layer 

 of material which makes them very resistant to the action of heat. 

 The name " bacteria " is commonly used as a general name for the 

 whole group. The name " micro-organisms " includes also yeasts, 

 moulds, and small animal organisms, protozoa, of which amoeba is 

 one. Protozoa are abundant in the soil and in stagnant water. 



Those bacteria concerned in the destruction of animal and plant 

 remains are the cause of what is known as "putrefaction" the final 



