218 GROWTH OF STEMS 
become active in the manufacture of food, often have a dry 
weight less than that of the seed. 
Temperature is the most important factor in growth and, just 
as in the germination of seeds, the minimum, maximum, and 
Fic. 198. — Two Potato plants, one of which was grown in the dark and 
the other in the light. A, plant grown in the dark. B, plant grown in the 
light. After Pfeffer. 
optimum temperature for growth vary with the kind of plant 
as shown in the table on next page. For example, the optimum 
temperature is between 90° and 95° for Corn, between 80° and 
85° for Barley, and about 70° for White Mustard, one of the 
weeds. Thus when the days and nights are so cool that such 
