INTRODUCTION 47 



action of light, to form starch in the chloroplasts. 

 A state of turgescence assists the production of 

 starch. Light and temperature must be of a more 

 or less standard character, assimilation being weak 

 or ceasing in a poor light or at a low temperature. 



The process of carbon assimilation after photo- 

 synthesis is one of the most important in the whole 

 of the plant's activities. Upon its success depends 

 also the possibility of animal life. 



g. The Food Requirements of Plants. The 

 Materials for Transportation, Elabora- 

 tion, ETC. 



If a plant be burnt there will be found on exami- 

 nation of the ash that remains behind after combus- 

 tion some thirteen chemical elements. These are — 

 carbon, hydrogen, oxygen, nitrogen, sulphur, phos- 

 phorus, potassium, calcium, magnesium, iron, sodium, 

 chlorine, silicon. 



Some of these are, moreover, found in the sub- 

 stances built up by the plant, such as starch, into 

 which carbon, hydrogen, and oxygen enter, as repre- 

 sented by the chemical formula CgHjoOg. In the 

 proteids also are found nitrogen and sulphur. Phos- 

 phorus is found in the nucleus, and was at one time 

 regarded as the physical basis of life. 



An experiment which proves that these elements, 

 or some of them, are essential to plant life is the 

 growing or culture of plants or seeds in a solution. 



