26 THE DEVELOPMENT OF RAW COTTON 



Similar poisoning from overheating takes place when 

 seed is sown too late into the summer. Otherwise the 



The Tern- ro ^ rare ty suffers from this cause, being 

 perature of buried in the cooler soil ; but the stem is 



the Plant. o ft en affected on hot days, when the temper- 

 ture exceeds 37 C. (Fig. 4). It must be remembered 

 that the actual temperature of the stem tissues is the im- 

 portant thing in this respect, and not the shade air tem- 

 perature, which may be widely different, the plant having 

 a lower temperature than the air, through evaporation 

 from the leaves, when there is ample water round the roots, 

 and a higher temperature if the roots are short of water. 

 The presence or absence of wind, clouds, newly watered 

 soil, etc., also affect the temperature of the air itself in 

 the cotton-fields, and may make it quite different from 

 the temperature recorded in a meteorological screen 

 close by- 



We have seen, then, that a rise of tissue temperature 

 up to about 33 C. continually accelerates growth, but 



that prolonged exposure to higher tempera- 

 Limiting tures than thi ig pre j udicial . TMs u 

 Factors. 



assumes that light, water-supply, etc., are 



sufficient. If any one of these is deficient, the growth 

 will accelerate with rise of temperature until it reaches a 

 rate at which it is using all the food or all the water 

 available, and beyond this rate it plainly cannot pfiss, 

 even though the temperature continues to rise; in other 

 words, food or water becomes the "Limiting Factor" 

 instead of temperature (Fig. 3). 



The relations of the seedling to water bring out one or 

 two points of interest, which bear upon our previous con- 



