112 GROWTH AND WORK OF PLANTS 



178. Amount of carbohydrates formed. According to 

 some experiments by Sachs,* the increase in dry substance of 

 leaves of a sunflower or squash was about 20 grams per day of 

 twelve hours on bright, warm days, for one square meter of leaf 

 surface. He estimates that on a warm, bright day a vigorous 

 sunflower would make an increase in dry weight of about 36 

 grams, and a squash about 185 grams dry substance. This 

 included other things than starch. Brown and Morrisf estimate 

 that in one day (twelve hours) one square meter of leaf surface 

 of the sunflower will make about 8J grams of carbohydrates of 

 which the larger part is sugar, the smaller part starch. This 

 would make, on the same estimates as above, about 15-20 grams 

 of carbohydrate as the result on one day's work of a vigorous 

 sunflower on a warm, bright day. 



179. Amount of carbon dioxide used during photosyn- 

 thesis. J Large quantities of carbon dioxide are used in photo- 

 synthesis. A sunflower plant uses about 50 grams carbon 

 dioxide per day (= nearly 2 oz.). Supposing the earth were 

 covered with sunflowers it has been estimated that they would 

 consume 135,000,000,000 kilograms (=297,000,000,000 Ibs.). 



to the plant. With suitable temperature conditions, the products of photo- 

 synthesis increase with the increase of the intensity of light, from very weak 

 light where photosynthesis is feeble, to the brightest sunlight where it reaches 

 its highest intensity. Temperature also influences the rate of photosyn- 

 thesis. At low temperatures it is feeble or nil, and increases up to 25 C.- 

 40 C. (77 F.-IO4 F.) where it reaches its highest intensity, and with higher 

 temperatures soon ceases. Photosynthesis also takes place at quite low tem- 

 peratures even several degrees below freezing. Photosynthesis continues in 

 winter mostly in evergreens at freezing or even a few degrees below, but quite 

 low temperatures bring about inactivity of the chlorophyll bodies. The 

 number and distribution of stomates also conditions the rate of photosyn- 

 thesis since the diffusion of gases is dependent on them. When the plants 

 are quite dry the rate is less than when the leaves are well supplied with 

 water, other things being equal. 



* Arb. bot. Inst. Wiirzburg, 3, 1884. 



f Journal Chem. Soc., 63 (Transact.), 604. See Jost, Pflanzenphysiolo- 

 gie, 140, 1904; p. 114, Engl. Ed., 1907. 



J For reference. 



