164 THE LOWEST ORGANISMS AND THE CELL 



(carbohydrates) by photosynthesis. This makes carbon, obtained 

 from the carbon dioxide of the air, available in these primary 

 foods. Nitrogen is obtained from the nitrates dissolved in water 

 and drawn up through the roots, and sulphur and phosphorus in 

 a similar manner from sulphates and phosphates. The proteids 

 of protoplasm are built up from these elements, with the addi- 

 tion of hydrogen and oxygen. Plants are able to form some 

 very complex organic substances, but animals are able to carry 

 the building-up process still farther, for the highest forms of 

 proteids known are found in their substance. 



There is, however, a turning point in the building-up activi- 

 ties when complex compounds begin to break down into simpler 

 substances. Some of these are the daily waste products of 

 an animal or plant. The most striking phenomena are those 

 which occur during the processes of decay, which begin at once 

 with the death of an organism. Decay is the process by which 

 highly complex organic compounds are broken down into suc- 

 cessively simpler substances. The final steps return the ele- 

 ments carbon, nitrogen, sulphur, and phosphorus to the earth 

 and air in very simple forms available again for the constructive 

 work of green plants. The processes of decay are due to the 

 growth and activities of bacteria and other fungi, and the sub- 

 ject is treated at some length in Sec. 252. 



199. Cell division, growth, and reproduction. Assimilation 

 increases the amount of protoplasm, and this results in growth 

 and reproduction through cell and nuclear division. Cell divi- 

 sion in plants, as in animals, is preceded by nuclear division, 

 after which a cell wall is formed between the daughter pro- 

 toplasts. The nucleus in the resting condition contains granular 

 material called chromatin, which may be readily colored (stained) 

 by certain dyes. Generally there are also present one or more 

 globular bodies called nucleoles (Fig. 170, A). Chromatin is a 

 proteid and is believed to be the essential substance of the 

 nucleus and necessary for the life of the cell, because protoplasm 

 will not live if deprived of nuclei. Just previous to nuclear 



