RATE OF PRODUCTION OF CARBOHYDRATE 209 



sunlight was considerably stronger and the increase of 

 weight was found to be I • 7 mg. 



Rate of increase of weight, 



under midday sun . 1 • 10 mg. per hour . (3) 



It is thus seen that, while under light from the sky the 

 rate of production of carbohydrate of a given specimen 

 was 0-24 mg. per hour, the rate is increased to i-i mg., 

 or nearly 5 times, under the light of the midday sun. The 

 above results show how the formation of carbohydrate 

 may be directly determined in the living plant by the 

 method of weighing the submerged plant. 



Determination of the Rate of Production of Carbo- 

 hydrate per Unit Area of Photosynthetic Surface 



In the three examples given above, the total area of the 

 leaf-surface in each case was 8 sq. cm. The following are, 

 therefore, the rates of formation of carbohydrate per square 

 metre of the surface per hour. 



Diffuse sky light — Rate per sq. m. per hour . 0-30 grm. 

 Sunlight in forenoon ,, ,, . 1-08 ,, 



Midday sun „ „ .1-30 „ 



In the last two cases sunlight struck the leaves in a 

 slanting direction. It has been shown (p. 47) that the rate 

 of photosynthesis is at its maximum at perpendicular 

 incidence of light. This condition was secured by means 

 of a reflecting mirror, and the determination of increase of 

 weight was made by the more sensitive method of evolution 

 of oxygen to be fully described in the next chapter. The 

 maximum rate of formation of carbohydrate in the leaf of 

 Hydrilla under sunlight at perpendicular incidence was thus 

 found to be 2*8 grm. per hour per square metre of the leaf- 

 surface. 



