CHAPTER XX 

 PHOTOSYNTHESIS 



The dry matter content of any plant tissue can be determined with a 

 fair degree of accuracy by drying a sample of that tissue in a suitable oven 

 at a temperature of iOO° C. The residue remaining after evaporation of 

 the water represents the non-aqueous constituents of the tissue. The per- 

 centage dry matter content of plant tissues varies greatly, ranging from 90 per 

 cent or even more in dormant structures such as seeds to 5 per cent or some- 

 times less in very succulent tissues. That the dry matter fraction of any 

 plant tissue is composed principally of organic compounds can be demonstrated 

 by subjecting it to combustion. This is accomplished by transferring a sample 

 of the dry matter to a crucible and heating it over a flame or in a muffle 

 furnace at a temperature of about 600° C. The small grayish residue result- 

 ing from this treatment is called the ash, and represents roughly the mineral 

 salts which have been absorbed from the soil (Chap. XXIV). Almost all 

 of the dry matter is oxidized at this temperature and the decomposition 

 products pass off in the form of gases. Practically all of the dry matter which 

 disappears during combustion represents organic compounds which are decom- 

 posed as a result of subjection to high temperatures. The ash content of 

 plant tissues varies considerably, but usually lies within the range of i to 

 15 per cent of the dry weight of the tissue. The origin of the organic sub- 

 stance of plants long remained an unsolved problem, but it has now been 

 known for many years that it is synthesized by the plant from a limited num- 

 ber of simple compounds which are absorbed from the soil and the atmosphere. 



Photosynthesis as a Process. — The process in which simple carbo- 

 hydrates are synthesized from carbon dioxide and water by the chloroplasts 

 of living plant cells in the presence of light, oxygen being a by-product, is 

 generally called photosynthesis. On the continent of Europe and to a lesser 

 extent in Great Britain, the terms carbon assimilation and assimilation are 

 often used to designate this process. The common use of the word "assimila- 

 tion" to denote the process in which foods are incorporated into the structures 

 of the plant body (Chap. XXXI) makes the employment of this term or 

 carbon assimilation as a synonym for photosynthesis undesirable. 



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