130 



HUTCHIXSON'S POPULAR BOTANY 



Assimilation, indeed, is found to consist essentially in the decomposition 

 of carbon dioxide and the formation of some kind of sugar possibly glucose 

 (C 6 Hi 2 G ), possibly canose (C 12 H 22 O n ) in the chlorophyll corpuscles. Allusion 

 was made to this some pages back, where it was pointed out that the cells 

 containing chlorophyll which are always near the surface of the plant 

 absorb carbon dioxide from the atmosphere or water (the latter in the case 

 of submerged plants), and that this gaseous compound is decomposed in the 

 chlorophyll corpuscles under the action of light. It was also stated that the 

 first organic compound as a result of the process is, in most plants, a form of 



sugar.* The importance of the leaves 

 in the economy of Vegetable Life will 

 be seen at once, when it is added that 

 these are the organs chiefly concerned 

 in the work in question. 



Y T ou may illustrate the process by a 

 simple experiment. Let the stem of any 

 pond-weed of convenient size be placed 

 in water which holds carbon dioxide in 

 solution (a little spring water will be 

 pretty sure to contain a sufficiency for 

 the purpose), and exposed to sunshine. 

 What follows ? From the cut surface 

 of the stem, bubbles of gas are given 

 off at regular intervals. The liberated 



,^l_. -, If^?' }*</ bubbles consist of oxygen. Probably 



f&'V5rfy V:: .'"'' you now perceive what has taken place. 



Some of the carbon dioxide has been 

 absorbed by the leaves of the plant, 

 and there decomposed, under the influ- 

 ence of light, the oxygen having been 

 given back to the water as useless. This 

 setting-free or evolution of oxygen from 

 plants is popularly known as " breath- 

 ing " or respiration, but the term, in this 



application of it, is altogether erroneous, the process being one of exhala- 

 tion and simple evaporation. Plants do respire, just as animals respire,f but 



* It has been proposed to apply the term photosynthesis instead of assimilation to this 

 process. "As the activity of the chlorophyll apparatus is so essentially dependent upon 

 light," says Dr. Reynolds Green, "the process of construction of carbo-hydrate substances 

 from carbon dioxide and water, which is its primary object, may appropriately be called 

 photosynthesis. This term has certain advantages over the older expression, the assimilation 

 of carbon dioxide, as the term 'assimilation' may preferably be reserved for the process of 

 the incorporation of the food materials into the substance of the protoplasm " (Introd. to 

 Veg. Phys. 1900). 



t That is, by giving out carbon dioxide and watery vapour and inhaling oxygen. A plant 

 placed in pure carbon dioxide would soon be suffocated, just as would an animal. 



FIG. 165. HORIZONTAL SECTION 



THROUGH THE EPIDERMIS OF A 



YUCCA LEAF, 



Showing stomata. 



