[Chap. VI THE INTERPRETATION OF PLANT BEHAVIOR 55 



during the day, except on cloudy days. Another student found that the 

 stems of a scarlet runner bean that grew in the dark were 3.6 times as 

 long as those that grew in bright light. Upon further investigation he 

 discovered that one-third of the increased height in darkness was due to 

 the increase in the number of cells formed by cell division, and that two- 

 thirds of the increase was due to a greater elongation of cells. From these 

 facts we may conclude that the slower growth in height of stems in 

 bright light is a consequence of more restricted cell division and cell 

 enlargement. 



From physicists we learn that light does not affect plants unless it is 

 absorbed bv them. Hence the sprouts in the dark are not affected by 

 light, they are not attracted by it, nor is their growth in length influ- 

 enced by it. Light apparently affects many plants in a way that results 

 in a decrease in both cell division and cell enlargement. We are familiar 

 with the fact that light initiates chemical changes in photographic films 

 and that it increases the rate of evaporation of water from wet surfaces. 

 Similarly, when light ra\'S strike the molecules in plant cells they start 

 physical and chemical changes which may initiate a series of events and 

 consequences that finally results in decreased cell division and cell en- 

 largement and thus in a reduced growth in length of the stem as a whole. 



Evidently between the first action of light upon the cells and the 

 diminished growth in height of the stem, there may be a long series of 

 events and consequences. If we knew all the facts involved, we might 

 then be able to trace these events and consequences in the order of their 

 dependence upon each other. Some of the facts are known; but we must 

 be content for the present with the general statement that the slower 

 rate of cell division and enlargement during tlie day is often a conse- 

 quence of certain changes in cell processes related primarily to the 

 lower content of water in the cells. 



The inhibiting effect of light on the elongation of stems may also be 

 due to its influence on certain chemical compounds called growth sub- 

 stances, or hormones. Cell enlargement in particular is affected by these 

 compounds through their influence on the growth of cell walls. When a 

 plant is placed at a window, growth in length of the brightly lighted 

 side of the stem is checked and the tip of the stem bends toward the 

 window. In such instances the inhibiting effect of light is due primarily 

 to its influence upon the translocation and the reaction of growth sub- 

 stances on the lighted side of the stem. The relation of these substances 

 to light and the growth of cells will be described further in later 



