58 



EFFECT OF PHYSICAL FACTORS AND PLANT CONDITIONS. 



were made, 6 of which were discarded because too great a change of 

 temperature occurred while the stomata were opening or because 

 the stomata were not closed at the start. 



Just above freezing, it required 8 hours to produce opening. At 

 10C. approximately 4 hours were required to reach maximum. At 

 20 C. slightly less than 2 hours was required, and at 30 C. nearly an 

 hour. The highest temperature, 34 C., caused complete opening 

 in less than 50 minutes. At freezing and above 40 C. opening does 

 not occur or is very erratic. Hence, upper and lower limits for 

 stomatal movement must be near these temperatures. The results 

 obtained correspond with the time required for opening by the 

 plants of the series, these plants, however, requiring a somewhat 

 longer time, since the light intensity at the start is low. This also 

 varies, as apparently at higher temperatures it requires less light 

 to initiate opening. If this be true, it explains why plants show 

 opening at the first traces of approaching dawn when the weather 

 is hot, and no opening until after sunrise on cold mornings. 



The effect of changes in soil-temperature on stomatal movement 

 is difficult of determination for several reasons. During the course 

 of a 24-hour period, the superficial layer of dust in a cultivated field 

 undergoes greater changes of temperature than the air if the day is 

 clear, but at a depth of 1 dm. these fluctuations are not nearly so 

 great and consist of a steady, slow rise after sunrise and a gradual 

 decrease from evening through the night. The amplitude of this 

 variation increases as the soil becomes drier, since the conductivity 

 of dry soil is greater than that of moist. At greater depths this slow 



9 10 II MT. I 2 



FIG. 31. Series 34, factor data for September 10-11, 1919; temperature 2 dm. 

 above soil surface (A), at the surface (B), at 1 dm. in the soil (C). 



rise and fall becomes less, until at 5 dm. it is very slight. At such a 

 depth only a series of cold days following hot weather, or the reverse 

 of this, causes any distinct changes in temperature. Naturally, there 

 is a gradual rise at this and greater depths during the spring, and as 

 slow a decrease toward winter, but these seasonal changes do not 

 come within the scope of this discussion. The changes observed in 



