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 



Fig. 198. — Two Potato plants, one of which was grown in the dark and 

 the other in the hght. A, plant grown in the dark. B, ])lant grown in the 

 Ught. 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 



