THE LEAF 219 



the plants, are grape-sugar, found abundantly in all sweet fruits, 

 cane-sugar, found in Sugar-cane, cellulose, a carbohydrate like 

 starch and ^ugar, of which the cell-walls are built, oils and /«^s, 

 common in seeds (Castor-oil, Gingili), alkaloids w'ith either 

 medicinal or poisonous, or stimulating properties (Coffee, Tea), 

 and acids (Citron and many fruits). 



The products prepared in the cells of the leaves, are then 

 carried through the ribs and the stalk of the leaf and through 

 the stem to wherever their presence is required. They are also 

 stored up in the stem, in tubers, in bulbs, in roots, and especi- 

 ally in the seeds, as a reserve of material for future use. 



(/) Importance of Photosynthesis in the Household of Nature. — 

 Considering that by the absorption by plants of carbon dioxide 

 exhaled by animals the balance of the constituent parts of the 

 atmospheric air is maintained, and still more that by this process 

 food is prepared not only for the growth of plants, but also for 

 the maintenance of all animal and human life on earth, we un- 

 derstand of what immense importance the process of photosyn- 

 thesis is in the household of Nature. We can safely say that 

 without carbon-assimilation by plants there would be no organic 

 life, and without the light of the sun there would be no photo- 

 synthesis and, therefore, no life on earth. 



3. Respiration. 



(a) Proof of the Fact that Plants absorb Oxyg-en and set free 

 Carbon Dioxide. — Plants, like animals, absorb oxygen and give 

 off carbonic acid gas, which process is known as respiration. 

 This cannot usually be observed at the time when the process of 

 assimilation takes place. It is, however, very evident in parts 

 that are not green and in all parts at night. Take, for instance, 

 two narrow-necked glass bottles of equal size and till one of 

 them one-third with germinating seeds of Bengal gram or flow^er- 

 buds. After about a day insert a lighted taper. In the empty 

 bottle the taper will go out after it has burnt a little while, i. e., 

 until the oxygen contained in the bottle is used up by the burn- 

 ing taper. In the other one the taper goes out at once, showing 



