160 GREEN PLANTS AS FOOD MAKERS 



Practical Exercise 9. Make a collection of leaves showing as many color 

 changes as possible. 



Self-Testing Exercise 



Water rises in the stem of plants through (1) (2). 



The green leaf needs (3), (4), and (5) 



(6) in order to manufacture organic food. These materials 



enter the plants through the (7) in the leaves and through 



the (8) in the soil. Plants growing in total darkness are 



without (9) (10) (11). This material is 



known as (12). 



PROBLEM VIII. WHAT ARE THE PRODUCTS AND RESULTS 

 OF FOOD MANUFACTURE? 



Comparison of carbohydrate-making and milling. The manu- 

 facture of carbohydrate by the green leaf is not easily understood. 

 The process has been compared to the work of a mill. In this 

 case the mill is the green part of the leaf. The sun furnishes the 

 motive power, the chloroplasts constitute the machinery, and soil 

 water and carbon dioxide are the raw products taken into the 

 mill. The manufactured product is sugar which is later changed 

 into starch. A certain by-product (corresponding to the waste 

 in a mill) is also given out. This by-product is oxygen. To un- 

 derstand the process better, we must refer to the diagram of the 

 leaf (page 161). Here we find that the cells of the green layer 

 of the leaf, under the upper epidermis, perform most of the work. 

 The carbon dioxide is taken in through the stomata and reaches 

 the green cells by way of the intercellular spaces and by diffusion 

 from cell to cell. Water reaches the green cells through the veins. 

 It then passes into the cells and there becomes part of the cell sap. 

 The light of the sun easily penetrates the cells of the palisade layer, 

 giving the energy needed to make the starch. This whole process 

 is a very delicate one, and will take place only when external 

 conditions are favorable. Chlorophyll absorbs light of certain 

 wave lengths, the blue and red rays do most of the work in food 

 manufacture. For example, too much heat or too little heat stops 

 carbohydrate-making in the leaf. The leaf engine works rapidly 

 under favorable conditions and makes sugar in such quantities that 



