110 STRUCTURE AND FUNCTIONS OF LEAVES 



pumpkin or squash leaves produce starch at about the same rate. 

 In a summer day fifteen hours long they can make nearly three 

 quarters of an ounce for each square yard of leaf surface. A 

 full-grown squash leaf has an area of about one and one-eighth 

 square feet, and a plant may bear as many as a hundred of them. 

 The entire plant would then produce nearly nine and a half 

 ounces of starch per day. 



Another way to emphasize the amount of work done by the 

 leaves is to consider how much air would be needed to supply 

 the carbon in a given weight of wood ; for all this carbon has 

 probably been derived from carbohydrates made in the leaves 

 (or other green parts) by photosynthesis. If the wood of a tree 

 after drying weighs 11,000 pounds and is half carbon, the latter 

 would weigh 5500 pounds. Taking the carbon dioxide contents 

 of the air at j^, there would be not quite 27,000,000 cubic 

 yards of air needed to furnish the carbon of such a tree. 1 



The enormous amounts of carbon dioxide annually removed 

 from the air by the growth of plants are continually being 

 replaced by the respiration of animals, the decay of animal and 

 vegetable material, and by the burning of fuel. From the burn- 

 ing of coal alone it is estimated that nearly 3,000,000 million 

 pounds of carbon dioxide are every year returned to the 

 atmosphere. 



133. Respiration. Plants cannot carry on their life processes 

 without consuming oxygen and giving off carbon dioxide and 

 water. This oxygen consumption is the respiration of plants. 

 Like animals, plants are dependent on the union of oxygen with 

 oxidizable substances in their tissues for the energy with which 

 they do the work of assimilation, growth, and reproduction, 

 in other words perform their life processes. 



How oxygen can be made to combine with the carbon- and 

 hydrogen-containing compounds in the plant at moderate tem- 

 peratures is a problem which plant physiologists have not yet 

 fully solved ; but the union does constantly go on, and as a 



1 Taken with slight alterations from Peirce, Plant Physiology, p. 44. 



