THE PLANT 83 



temperature from the minimum, slowly at first, then more 

 rapidly, until the optimum temperature is reached, af t§r which 

 it diminishes rapidly to the maximum temperature. 



Now, in temperate climates the maximum temperature for 

 most plants is rarely, if ever, reached, and as a rule the growth 

 is greater the higher the temperature. Fortunately, too, in 

 such climates the daily range of variation in temperature is not 

 great. Thus during the growing season the plant is rarely 

 cooled to the minimum or heated to the maximum temperature, 

 and its growth will be greater the longer the time at which it 

 is near the optimum temperature. Moreover, the disturbi'.g 

 effect of excessive heat and cold will be absent. 



But in tropical countries, especially at considerable elevation 

 and far inland, the conditions are different. The daily range 

 of temperature is much greater. In the night, and especially in 

 the early morning, the ground temperature may sink below 

 the minimum, while in the hot midday sun it may rise con- 

 sidembly above the maximum temperature of growth. Conse- 

 quently plaijts, though they may pass through the optimum 

 teiiij)erature perhaps twice in the twenty-four hours, are kept 

 near that temperature for but a short period each day, and 

 even then are not able to take advantage of it, because of their 

 being disturbed by the rapidity of the change. It has been 

 alleged that in such countries — South Africa, for example — the 

 screening off of the early morning sun has a very injurious 

 effect upon many plants, and has led to the belief that there is 

 something peculiarly favourable to plant-life in the rays at 

 sunrise. But the influence can be explained by the tempera- 

 ture effect produced. The following experiment by the writer 

 will show this. In the case of a thermometer on the ground 

 Fcreened from the sunrise rays from 6.30 to 9.30 a.m. the 

 temperature, at first about 6' C, rose very slowly to about 16° 

 at 9.30, and then rapidly (after direct suni^hine fell upon it) to 

 28°, and subsequently gradually to 37° C. at 11 a.m. At the 

 same time a plant receiving the direct rays of the morning 

 sun rose from 6° C. at 6.30 a.m. to 11° at 7.15, attained 16° at 



