Relations of Plants Above the Ground 105 



for syrup or for forage; corn grown for ensilage or for 

 grain. 



152. The Vigor of Leaves and Stem Growth. The size 

 of leaves is influenced largely by the amount of water 

 available to the plants during the period of their for- 

 mation. From this, it follows that plants grown for their 

 leaves, like cabbage, lettuce, hay crops, etc., do best 

 when plenty of moisture is in the ground. Light is neces- 

 sary for the formation of leaves, as we have seen. Where 

 branches are shaded, the lower leaves are small and 

 weak, and often fall off before the season ends. As the 

 buds, from which the branches, leaves and flowers of the 

 succeeding season grow, are formed in the axils of the 

 leaves and take their vigor from them, it is important 

 that fruit trees be pruned out so that light may reach 

 to all parts. (See Chapter XVIII.) 



153. The Temperature of the Air is subject to great 

 and often sudden variations, whereas the soil, as we have 

 seen, changes its temperature very slowly. The above- 

 ground portion is more often injured by extreme cold 

 or excessive heat than the part below the ground. 

 The first effect of lowering the temperature is to retard 

 the growth of the plant. Cold does not permanently 

 affect all plants alike. Some plants are killed by moder- 

 ately low temperature, while others are uninjured even 

 by long exposure to severe freezing. The ill effects of 

 freezing are more severe on plants when full of sap. 

 Peach trees may endure a number of severe freezes 

 through the winter, but if a severe cold spell comes 

 late in the spring, after the buds have swollen, the 

 injury is often considerable. 



Sometimes the bad effects are due to the sudden 

 thawing, more than to the cold itself. The winter-killing 



