THE EFFECTS OF TROPICAL SUNLIGHT ON PLANTS. 



By D. T. MacDougal. 



EVERY botanist who visits the tropics is impressed with the 

 greater intensity of the sunlight and the numerous adaptations 

 of plants by which damage from such fierce insolation is 

 avoided. The very prominence of such adaptations generally 

 leads to the overestimation of both the intensity and amount of light 

 received from the sun in the tropics. According to Wiesner's re- 

 searches, the greatest intensity of the sunlight at Buitenzorg is 1.612 

 units against 1.5 units at Vienna. The total daily amount of light 

 received at Buitenzorg on days in November and December does not 

 exceed that at Vienna in August, and the daily amount at Buitenzorg 

 in January is equal to that of Vienna in June, the greater length of 

 the day in higher latitudes compensating for shorter day with fiercer 

 light in the tropics. 



These generalizations may be taken to hold good for moist tropi- 

 cal climates only. In such dry climates as that of the Liguiana plain 

 in Southern Jamaica, where the low humidity does not allow any great 

 atmospheric absorption of the more refrangible rays, the total daily 

 amount is probably greater than in average temperate latitudes. 



A recent writer, Dr. A. J. Ewart, has made a valuable contribu- 

 tion to the subject.* 



Dr. Ewart has found that the activity of chlorophyll is greatest 

 between 25 and 35°C, and that it varies to 4o°C and decreases steadily 

 from 4o°C to 5o°C. In normally exposed plants the optimum temper- 

 ature is maintained by the evaporation from an abundant supply of 

 moisture, and by varying the angle of the leaf -surf aces to the light by 

 pulvinar and passive drooping movements as well as by layers of red 

 or violet coloring matter. 



Thick leaves of shade-loving plants, such as Hoya, fastened in 

 such position that the sun shines perpendicularly on the surfaces, may 

 attain a temperature of 45 to 5o°C, and suffer injury in consequence. 



The necessity and importance of being able to move the leaves 

 rapidly and accurately has led to the development of motor organs, 

 pulvini, in many groups beside the Leguminosse, in which such struc- 

 tures are most highly developed. Pulvini are to be found in many of 

 the Monocotyledons, in which it is a greater variation than in Dicoty- 

 ledons. 



* The effects of tropical insolation. Annals of Botany, II. : No. 31, p. 439, 1897. 



