MORPHOLOGY OF LEAVES AND FLOWERS 601 



Transpiration. "We are familiar witb the fact tbart; the air is 

 continually taking up water in the form of vapor. ,A dish filled 

 with water and exposed to the air in the laboratory or out of doors 

 will soon become dry. We hang wet clothes upon the line so that 

 the ladr will take up the water which they contain. When we look 

 into the physics of this process, we find that the water is really the 

 active agent, and that it is continually changing into vapor and 

 passing into the air. This process of changing into vapor we call 

 evaporation, and its rate depends upon temperature, the amount 

 of moisture already present in the air, and atmospheric pressure. 

 This same process of evaporation goes on in the leaf, for the air 

 surrounds the leaf and fills the intercellular spaces within. This 

 continuous loss of moisture from the leaves is called tramspiration. 



As seen in photosynthesis, water must be present in the leaf cells 

 in sufficient quantity or the process of food making will be hindered. 

 Water is further needed for dissolving and transporting food ma- 

 terials. It is evident that transpiration is continually diminishing 

 this quantity of water which is so necessary and if the supply, which 

 is furnished from the ground through the roots and stems, does not 

 equal the loss, disaster will come to the plant. So far transpira- 

 tion seems to be only a detriment to the plant. It is thought to be 

 of use in that it increases the flow of the water from the soil and 

 through the plant and thus increases the amount and better dis- 

 tributes the salts secured from the soil. 



It is remarkable how well most weeds can thrive, when economic 

 plants are suffering severely from trpjispiration. This may be due 

 to protective modifications which cut down transpiration or to the 

 ability to supply the loss through a more efficient root system. 



One of the harmful effects of weeds is the taking from the soil 

 of the water which economic plants need. According to careful 

 estimates a sunflower {Eelianthus annuus) six feet high transpires 

 on the average about 1 quart per day. A grass plant has been found 

 to give off its own weight of water every ' twenty-four hours in 

 hot, dry summer weather. This would make about 61/2 tons per acre 

 or more than one thousand gallons every twenty-four hours for 

 ordinary grass fields, or rather about 200 gallons for a plot about 

 the size of a city lot. From the above figures we can form some 

 notion of the immense loss of water from the soil through weeds, and 

 see how weeds can retard the growth of economic plants. 



