154 GREEN PLANTS AS FOOD MAKERS 



in the text. In this diagram make arrows to show (a) how air gets to the 

 cells, (6) how water gets to the cells, and (c) how food materials made in the 

 palisade layer might get out of the leaf. 



Self-Testing Exercise 



A green plant (1) food in its (2). The 



(3) is fitted for its work by being (4) and exposing a 



(5) surface to the (6). It contains many small openings 



called (7), through which (8) passes. The size of 



the (9) is controlled by (10) (11). A leaf 



is made up of a (12), (13), and (14). 



PROBLEM VH. WHAT RAW MATERIALS AND CONDITIONS 

 ARE NEEDED TO MAKE FOOD? 



Demonstration 8. How does water get into leaves? 



Where is the passageway of water from the roots to the leaves? 

 Place a young growing pea or bean seedling in red ink (eosin) and 

 leave in the sun for a few hours. What happens? What happens 

 after a wilted plant is given water ? Why ? Place celery stalks in red 

 ink and leave for a few hours in the sunlight. Cut thin sections of the 

 stem. Where does the colored water rise? It is obvious from these 

 experiments that water rises through the minute tubes or ducts in the 

 stems. We will find later in most woodj^ stems that these bundles of 

 tubes are arranged in a very regular way. 



What raw materials are needed ? If we think back to our work 

 on foods in the last unit, we may remember that organic foods 

 consist of the elements carbon, oxygen, hydrogen, nitrogen, and 

 small amounts of certain elements found in the soil, such as 

 calcium, iron, potassium, and sodium. If the leaf is to manu- 

 facture organic food substances, then we must see where these 

 elements might come from. Water is made up of oxygen and 

 hydrogen ; carbon dioxide, a gas given off in the breath, is in the 

 air in small quantities, while nitrogen in a usable form is in soil 

 that contains humus. Here then are the raw materials. How 

 do they get into the leaf? 



We have just seen that water can get from the roots up through 

 the stem and into the leaves. This water, if it comes from the soil, 

 has dissolved in it mineral matter, including nitrates from which 

 the plant may obtain nitrogen. Carbon dioxide, which is taken 



