112 TEXTBOOK OF BOTANY 



It dissolves in the water of the cell and finally gets to the chlorophyll in 

 the chloroplasts (Fig. 34). 



Most of the oxygen liberated during photosynthesis passes from the 

 cells into the intercellular spaces of the chlorenchyma, then through the 

 stomates into the outer atmosphere. This passage of CO2 into, and of 

 O2 out of, the chlorenchyma also occurs when photosynthesis takes place 

 in green stems and fruits or in any other green part of the plant. The 

 sugar that is not locally consumed or transformed within the chloroplasts 

 passes from them to all other parts of the plant. The manner of its transfer 

 will be discussed in later chapters. 



Factors involved. Had the earlier investigators known that the increase 

 of Oi- and the decrease of COt- in the air surrounding green plant organs 

 exposed to light were the result of the synthesis of sugar, they could have 

 used their data to show that chlorophyll is necessary for this process. 

 The variations in the amount of CO2 and O2 in the air surrounding 

 leaves or other green plant tissues have been used in numerous experi- 

 ments in recent years not only to establish the fact of the necessity of 

 chlorophyll for photosynthesis, but also to study the effects of several 

 environmental factors upon the rate of photosynthesis. 



From the facts discussed thus far, one would expect sugar to be made 

 in plants only when the following are present: chlorophyll-containing 

 cells, water, carbon dioxide, light, and a suitable temperature. If any 

 one of these factors is absent, no sugar will be made, and the manufac- 

 ture of other foods from sugar cannot long continue. If the plant is con- 

 tinuously deprived of either light or carbon dioxide it will starve to death 

 when all the food that has accumulated in it has been used. If deprived 

 of water, it will die of desiccation before the accumulated food is con- 

 sumed. The drying of freshly cut hay and fodder, which are later used 

 as a source of food for animals, is a good illustration of the death of plant 

 organs by desiccation before the food within them is consumed by the 

 plant. 



In experiments it is possible to expose a plant to a temperature in 

 which it will consume sugar faster than it makes it. Plants in the field 

 are frequently exposed to such temperatures for short periods of time. 

 Various investigators have concluded that this condition accounts for the 

 limit of distribution of particular plants in certain geographic areas. As 

 we shall see later, it greatly affects the amount of growth of certain plant 

 organs. The death of plants exposed to extreme temperatures is more 



