26 PLANT PHYSIOLOGY 



is removed the water becomes more alkaline, and it will hence 

 take on a decided pink tinge if placed in sunlight. 



Lastly, among the easy demonstrations should be mentioned 

 the direct test for starch with iodine. Test plants for starch 

 which have been kept in the dark and in the light. Those in the 

 dark show little or no starch depending upon the length of time 

 they have been left there, while those in the light turn blue when 

 iodine is applied showing the presence of starch. If geranium 

 or other leaves are tested at the end of one, two, and three hours 

 after being brought into the light, they will show a progressively 

 increasing amount of starch present. 



Amount of Carbon Dioxide Consumed. — In 10,000 liters of 

 air there are only 3.3-3.5 liters of carbon dioxide. This weighs 

 about seven grams but only three-elevenths is carbon. In 10,000 

 liters of air one finds therefore only two grams of carbon. An 

 ordinary recitation room is about 9 X 11 X 4 meters, or it has a 

 volume of about 400 cu. m. This means 400,000 liters, which 

 would contain about 80 g. of carbon or enough for 175 g. of starch. 

 An average sized oak tree would hence deprive 12,000,000 cu. m. 

 of air of carbon dioxide to get the requisite amount of carbon 

 stored up in the cellulose of its woody tissues. This means a 

 volume 30,000 times the size of the above-mentioned recitation 

 room. 



It has been computed that clover assimilates 63 mg. of carbon 

 dioxide per sq. m. per hour and that squash leaves assimilate 25 

 g. of sugar per sq. m. of leaf surface in 15 hours; while the catalpa 

 has been found to manufacture 3 g. of starch per sq. m. per hour. 

 At this rate it would take about 25 sq. m. of leaf surface to use 

 up the carbon dioxide given off by the average man in breathing 

 during 10 hours, or if computed on the basis of 24 hours, about 

 60 sq. m. 



Three hundred bushels of potatoes on an acre means about 

 2,500 kg. of dry matter including tops and roots. In terms of 

 starch, this would mean that all the carbon dioxide would be 

 used over the acre to the height of a mile and a third, assuming 

 an even distribution of the gas. It has been estimated that during 

 the growing season the average leaf produces enough sugar to 

 cover itself a millimeter thick, and that the plants of the United 

 States manufacture a cubic mile of sugar each year. 



Also it has been calculated that the wheat plant produces 



