THE LEAF AND ITS FUNCTIONS 73 



6CO2 + 6H2O = CeHioOe + 6O2 



Glucose is present in the sap of practically all plant cells. 

 It is the fundamental carbohydrate and the basis for all other 

 foods; and from it are ultimately derived, through the action of 

 enzymes and by various additions and chemical modifications, all 

 the organic compounds of plants and animals. 



The presence of a large amount of sugar in a chlorophyll- 

 bearing cell results in a stoppage of its manufacture there, and 

 is disadvantageous for other reasons. We find, accordingly, that 

 before photosynthesis has long continued, the resulting sugar 

 becomes converted into another type of carbohydrate, starch 

 (CeHioOs)!!.* Starch is complex and insoluble, occurring in 

 minute but definite bodies or grains, the size, shape and markings 

 of which are characteristic and constant in any plant species. 

 The starch molecule is very large — just how large we do not know 

 — and is derived through the combination of many glucose mole- 

 cules, with the liberation of a molecule of water from each, thus : 



nCeHisOe - nHsO^ {CR^.O,)^. 



Neither sugar nor starch are accumulated in very great quanti- 

 ties in the leaf-blade, for most of the products of photosynthesis 

 are removed shortly after their production to those regions of the 

 plant where they are to be used or stored. 



By-product.- — In the recombination of the atoms of carbon 

 dioxide and water out of which glucose is produced there is evi- 

 dently a surplus of oxygen, and we find this oxygen given off as a 

 by-product of photosynthesis, passing forth into the air continu- 

 ally from green plants during daylight. This is of little signifi- 

 cance to the plant itself but is often important to other organisms. 



Photosynthesis is, therefore, a constructive process by which 

 the food oj the plant is manufactured from very simple inorganic 

 materials, ' through the agency of the characteristic substance 

 chlorophyll, and by energy derived from fight. The significance 

 of photosj'nthesis lies in the fact that it is the only process among 

 living things whereby organic compounds are built up from simple 

 inorganic substances, with the resultant storage of energ}^ All 

 other chemical changes in plants and animals are concerned 

 either with the transformation of one type of organic material 



* n stands for the unknown number of smaller molecules which arc united 

 into one of the large and complex ones of such a substance as starch. 



