CHAPTER IV 



ASSIMILATION 



Photosynthesis can very easily be demonstrated. If, for 

 example, an aquatic plant, elodea, is placed in the sun with a 

 Uttle carbon dioxide dissolved in the water, bubbles are 

 almost immediately formed among the leaves — bubbles which 

 grow, detach themselves from the branch and burst on 

 reaching the surface, discharging their content of oxygen. 

 During this time, glucosides are synthesized in the leaf and 

 the grains of starch which have been formed can easily be 

 coloured blue with a drop of iodine solution. 



To show that this starch is the direct result of photo- 

 synthesis, the plant is left for a certain time in darkness until 

 it has used up all its stock; after ten minutes in sunlight the 

 leaves have obtained a new supply. 



Although it is easy to see, photosynthesis is difficult to 

 measure. There are three possible methods: to measure the 

 carbon dioxide absorbed, the oxygen given off, or the gluco- 

 sides accumulated. 



The greatest obstacle to precision is respiration, which 

 releases carbon dioxide, absorbs oxygen and destroys gluco- 

 sides. If these disturbing effects could be measured once and 

 for all, it would be sufficient to take them into account in 

 the calculation of the results. In absolute darkness photo- 

 synthesis does not take place, and only respiration is con- 

 cerned in the gaseous exchanges of the plant, but respiration 

 is not a constant phenomenon and temperature changes 

 cause it to vary; the quantity of glucosides also influences it, 

 as do many other factors which are often difficult to estimate 

 at their just value. When the plant is assimilating to the 

 maximum in full sunshine, can it be assumed that respiration 



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