36 



General Botany 



during photosynthesis. That this actually happens may easily 

 be shown by inverting under water a bundle of the branches of 

 some water plants, like Elodea, with the cut ends placed under 

 the mouth of a test tube that is filled with water. When exposed 

 to the hght for a day, the tube will be partly filled with gas. By 



testing with a glowing match or 

 splinter (Fig. 21), the gas may be 

 shown to be mostly oxygen.^ 



How the supplies are obtained. 

 Every industrial workshop must con- 

 stantly be provided with the raw 

 materials needed in the manufacture 

 of its product. Likewise the leaf 

 must be supplied with the substances 

 that it uses in the making of food. 

 These necessary supplies come to the 

 leaf through the veins and the 

 stomata. The water passes into the 

 leaf through the water-conducting 

 tissue of the veins. The supply of 

 carbon dioxide reaches the cells of 

 the mesophyll through the stomata 

 and the intercellular spaces. When 

 the stomata are closed, little or no 

 carbon dioxide can enter, and at such 

 times the process of photosynthesis is 

 of necessity greatly retarded or completely stopped. 



That the carbon found in a plant does not come from the soil 

 was shown 300 years ago by one of the earliest students^ of plant 



Fig. 21. Experiment to show the 

 giving off of oxygen from a water 

 plant {Elodea) during photosyn- 

 thesis. 



^ Water containing a considerable amount of dissolved carbon dioxide should 

 be used in this experiment so that photosynthesis may go on rapidly. Pond 

 water is better than tap water. 



2 Van Helmont (15 77-1644) grew the branch of a willow tree for 5 years. x\t 

 the beginning it weighed 5 pounds, at the end 164 pounds. The loss in weight 

 of the soil was 2 ounces. 



