104 GROWTH AND WORK OF PLANTS 



synthesis cannot take place in them. They are dependent for 

 their carbohydrate food on green plants.* They obtain it either 

 as parasites on living green plants, or by growing on their dead or 

 disintegrated remains as saprophytes; or on other plants or animals 

 which in their turn are dependent on the green plants primarily 

 for their carbohydrate food. 



173. Conditions favoring photosynthesis. From the pre- 

 ceding experiments and discussion it is clear that light and air, the 

 chlorophyll, and the living substance of the plant, are essential to 

 the formation of sugar and starch. But the rapidity of their for- 

 mation is influenced by the varying intensity of light, tempera- 

 ture, and the percentage of CO 2 in the air.f 



174. Amount of carbohydrates formed. According to some 

 experiments by Sachs, t 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 



* In using the term green plants here, the chlorophyll-bearing plants 

 are understood. It should be remembered that there are green plants, 

 especially among the fungi, which do not possess chlorophyll. 



f The products of photosynthesis increase, other things being equal, 

 with an increase of CO 2 from the normal (about .05 part in 100 of air) up 

 to 4 per cent (4 parts to 100), but a larger increase in the CO 2 acts injuriously 

 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 whexc 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. 



J Arb. bot. Inst. Wlirzburg, 3, 1884. 



