SECTION III. PHYSIOLOGY 

 CHAPTER VII 



NUTRITION AND SEASONAL LIFE OF PLANTS 

 PHOTOSYNTHESIS AND RESPIRATION 



The student has now become familiar with the form of plants, 

 and with their adjustments to the environment. He has also 

 studied the structure of the organs and tissues of the plant body, 

 upon which a proper understanding of its various functions is 

 based. The following discussion will therefore be confined to the 

 main physiological processes concerned with nutrition ; namely, 

 photosynthesis, respiration, transpiration, assimilation, and the 

 digestion of foods. 



PHOTOSYNTHESIS 



If a green leaf which has been exposed to bright sunlight for 

 a few hours is bleached with alcohol and then tested for starch 

 with iodine, the green mesophyll areas will be tinted blue or 

 bluish black. This will be found to be due to minute starch 

 grains, which are formed in the green chloroplastids of the 

 mesophyll cells during the exposure of the leaf to the sunlight. 

 The starch grains are formed in such numbers throughout the 

 green tissues of the leaf that the latter is tinted, owing to the 

 reaction of the iodine on the abundant starch. 



This simple experiment indicates the principal function of the 

 leaves, which is to manufacture sugar and starch for the plant. 

 This sugar and starch then becomes a basis for the manufacture 

 of other foods, such as the fats and the nitrogenous foods, or 

 proteins. It is probable that the leaf is the main center for the 

 compounding of these nitrogenous foods from the sugar which 



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