198 THE PLANT WORLD 



Self-Testing Exercise 



The (1) are plants which are either parasites or saprophytes, 



the latter living on (2) (3) matter and the former on 



(4) plants and animals. The (5) fungi do enormous 



damage to (6) every year. The black stem wheat rust, which 



lives on two (7), (8) and (9), does a yearly 



damage of almost $55,000,000. The pine tree (10) 



(11) is another recent importation from (12). It lives on two 



hosts : the (13) or (14), and the (15). To 



curb the damage from such parasites one of the (16) plants 



must be destroyed. 



PROBLEM VI. WHAT ARE MOLDS? WHAT DO THEY DO? 



Demonstration 5. To determine the conditions favorable for the 

 growth of mold. Place pieces of bread in each of four wide-mouthed 

 bottles or jars. Add a little water, and expose all four bottles to 

 the air of the living room or kitchen for half an hour. Then cover the 

 bottles and plunge one into boiling water for a few moments. Place 

 this and a second jar side by side in a moderately warm room. Place 

 the third jar in the ice box and the fourth in a hot dry place. 



Notice day by day any changes that occur in the contents of the 

 jars. In which jar does growth appear first? Do all jars have a like 

 growth of mold at the end of a given period of time? 



How does the mold get on the bread? Where does it come from? 

 Why did you add water to the jars? What conditions must you have 

 for the growth of mold? Conversely, how would you keep molds 

 from getting a foothold on foods ? 



Physiology of the growth of mold. Molds, in order to grow 

 rapidly, need food, darkness, oxygen, moisture, and moderate heat. 

 They obtain their food from the materials on which they live. 

 This they are able to do because they have rhizoids l which give 

 out digestive enzymes which change the starch of the bread to 

 sugar and the protein to a soluble form which can be absorbed by 

 the cells. These absorbed foods are then used to supply energy 

 and make protoplasm. Thus molds act like animals, except that 

 digestion takes place outside of the body. 



What can molds live on ? Molds feed upon all cakes and breads, 

 upon meat, cheese, and many raw vegetables. They are almost 

 sure to grow upon flour if it is allowed to get damp. Jelly and 

 other foods containing moisture are particularly favorable to the 



1 Rhizoids (ri'zoids) : rootlike filaments or threads. 



