34 HORTICULTURE FOR SCHOOLS 



by the plants, but in the daytime, when both respiration and 

 photosynthesis are taking place, all the carbon dioxide 

 liberated by respiration is used in photosynthesis and does 

 not get back into the air. In addition, a large quantity 

 of carbon dioxide is taken out of the atmosphere and a 

 considerable quantity of oxygen is added to the air. On the 

 whole, then, plants take carbon dioxide from the air and add 

 oxygen to it. 



It is of interest to note that the plant obtains hydrogen 

 from the water in the soil taken up by the roots; that oxygen 

 is secured from this source and from the air taken in by the 

 leaves; that carbon is taken in by the leaves from the air; 

 that nitrogen must be secured from the soil in the form of 

 salts by all plants except the legumes which, by means of the 

 bacteria living in nodules on the roots, are able to use the 

 nitrogen of the air; and that phosphorus, potassium, 

 magnesium, sodium, calcium, iron, chlorin, and sulfur must 

 be taken from the soil in the form of salt compounds. 



EXERCISES 



EXERCISE I. Exercises in plant structure. 



1. Materials. Various plants with their roots. 



Procedure. Classify the plants according to their root systems into 

 those possessing the following: (a) fibrous roots, (b) tap-roots, (c) 

 fascicled roots. Arrange your results in the form of a table. 



2. Materials. Plants; shrubs; small trees with their roots. 

 Procedure. Dig up carefully various plants and if possible small 



trees. Examine them for root-hairs. Notice especially their number 

 and where they are borne. 



3. Materials. Exogenous and endogenous stems; microscope. 

 Procedure. (a) Examine longitudinal and cross-sections of exogenous 



stems. Study their general structure. Draw and label the parts. 

 Make thin sections of young stems and examine them under a micro- 

 scope for cells of various shapes, (b) Examine longitudinal and cross- 

 sections of endogenous stems. 



4. Materials. Leaves: microscope. 



