140 GREEN PLANTS AS FOOD MAKERS 



minerals. These are calcium, sulphur, iron, potassium, magne- 

 sium, phosphorus, sodium, and chlorine. 



That plants will not grow well without certain of these mineral 

 substances can be proved by the growth of seedlings in a so-called 

 nutrient solution. If certain ingredients are left out of this solu- 

 tion, the plants placed in it will not develop into adult plants. 



Practical Exercise 2. Make a table in which you indicate the relative 

 amount of water that can be held by different kinds of soils. 



What kind of soil would you expect to find in a desert ? Covering the forest 

 floor? In a river valley? 



Self-Testing Exercise 



Humus or (1) containing (2) (3) will 



hold (4) much better than inorganic soil. When water 



passes through the (5), it takes out certain mineral salts 



which it holds in (6). Such water is called (7) 



water. 



PROBLEM II. WHAT FACTORS INFLUENCE THE GROWTH 



OF ROOTS? 



Root system. Examine the root of a bean seedling grown in 

 sawdust. The long main root is called the primary root. Other 

 smaller roots which grow from the primary root are called secondary, 

 and the roots growing from the latter are called tertiary roots or 

 rootlets. What functions do these roots appear to have? Most 

 of the roots examined take a more or less downward direction. 

 Does gravity act on the growing root? This question may be 

 answered by a simple experiment. 



Demonstration 2. To show the effect of gravity on a growing root. 



Plant mustard or radish seeds in a pocket garden. A very convenient 

 form of pocket germinator may be made in a few minutes in the following 

 manner : Obtain two cleaned four by five negatives (window glass will 

 do) ; place one flat on the table and on it place half a dozen pieces of 

 colored blotting paper cut slightly smaller than the glass. Now cut four 

 thin strips of wood so as to fit on the glass just outside of the paper. 

 Next moisten the blotter, place on it some well-soaked radish or mustard 

 seeds or grains of barley, and cover it with the other glass. The whole 

 box thus made should be bound together with bicycle tape. Seeds will 

 germinate in this box, and with care may live for two weeks or more. 



