210 General Botany 



plant. The synthesis of carbohydrates and other organic com- 

 pounds in the plant depends directly or indirectly upon sunlight. 

 As we shall see later, the relative amount of carbohydrates pro- 

 duced in the plant determines in the main whether the plant 

 continues vegetative growth or carries on reproduction. Light 

 also greatly affects transpiration, directly through raising the 

 temperature of the plant and indirectly by causing the stomata 

 to open. 



Atmospheric water. The water in the air affects plants directly 

 in several ways. The moistness or dryness of the air determines 

 whether less water or more is required for transpiration, and the 

 amount of water precipitated from the air in the form of rain 

 determines to a large extent the amount of water available in the 

 soil. Atmospheric water condensed in the form of fog and cloud 

 reduces transpiration and also lessens the amount of light that 

 reaches the plant. Under conditions of high humidity and 

 favorable temperature vegetative activity reaches its maximum. 

 Drought greatly decreases vegetative growth and shortens the 

 vegetative period of plants. A high rate of transpiration may 

 not only prevent any increase in the size of a plant during the 

 daytime but may actually bring about a decrease. 



Distribution of rainfall. The distrihution of rainfall through 

 the year is of the greatest importance to vegetation. When the 

 period of heaviest rainfall coincides with the hottest part of the 

 year, the conditions are best for the rapid growth of plants. If 

 the rainfall is scanty during the time of highest temperatures, 

 plants are hindered in their growth, and only xerophytes may be 

 able to withstand the conditions. In these regions irrigation is 

 absolutely necessary for the growth of mesophytes. The greater 

 amount of available sunlight in summer-dry regions accounts in 

 part for the unusually large crops that can be raised on the 

 irrigated lands of the Western states. This is one of the principal 

 reasons why California has become an important center of the 

 production of flower and vegetable seeds. 



