110 PLANT PHYSIOLOGY AND ECOLOGY 



but grow little; in the other division is greatly diminished or 

 altogether lacking, and the stretching of the cells pronounced. 

 The former consists almost wholly of meristem and may be called 

 the zone of division, while the latter consists of cells more or less 

 modified into parenchyma, and may be termed the zone of elonga- 

 tion. The zone of division is regularly much shorter than that 

 of elongation, the ratio often being greater than 1 : 3. The zone 

 of division occupies the tip of stem or root, and the zone of elon- 

 gation extends back from it for a distance several times greater. 

 Elongation is greatest just back of the meristem, and it decreases 

 gradually toward the permanent tissue of the organ. In the 

 intercalary growth of an onion leaf, elongation is greatest at the 

 apex of the sheath, and it decreases less rapidly upward into the 

 blade and more rapidly downward. The length of the growing 

 region varies for different plants. In roots it may vary from 

 one to several centimeters. The growing region of the stem is 

 normally much longer, ranging from 5-40 centimeters and in 

 rare cases even more. It may consist of one to several inter- 

 nodes, and in certain water plants growth may extend over as 

 many as fifty internodes or rarely over the entire stem. 



Experiment 36. Influence of temperature, water, and light upon regions 

 of growth. Germinate seeds of Vicia faba in a moist chamber, and as 

 soon as the roots are a centimeter long, mark several carefully with an 

 inked thread at intervals of a millimeter. Put one or two in a warm place 

 and as many in a cool spot Determine the region of greatest growth, 

 and note the differences due to temperature. 



Select four sunflower seedlings that show three internodes, taking 

 pains to choose plants as nearly alike as possible. Mark the stem of 

 each at 5 mm. intervals, and mark one of the youngest leaves of each 

 transversely at intervals of 2 mm. Place one plant in a warm sunny 

 place and keep it well watered, marking the other leaf of the upper 

 pair longitudinally at intervals of 2 mm Place the second plant along 

 side the first, but do not water it. Put the third plant in darkness, 

 and the fourth one in a cool spot. Follow the growth in each from day 

 to day. After several days or a week determine the total growth of 

 stem and leaf under each condition, as well as the region of greatest 

 growth for stem and leaf. 



133. Rhythm of growth. The rate of growth is by no means 

 uniform throughout the day or the year. It exhibits a certain 

 rhythm or periodicity, in accordance with which the growth flue- 



