Factors Affecting Growth and Reproduction 219 



Sulfur forms a part of all plant proteins. It also occurs in 

 certain compounds common in the mustard family, that give 



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d; 







GRAVITY 



4' V V V V V V 



Fig. 126. Diagram showing the principal factors in the environment of land 



plants. 



them their pungent odor and taste. Onions, garlic, and leeks, 

 members of the Hly family, owe their flavors to sulfur compounds. 



Iron is essential for the development of chlorophyll, although 

 it forms no part of the chlorophyll molecule. It appears to 

 function chiefly as a catalyzer in the plant. 



Phosphorus is a necessary element in certain compounds found 



