AN INTRODUCTION TO PLANT BIOLOGY 107 



Effect of Girdling. If a rose bush or other shrub be 

 girdled (cutting away a ring of bark down to the cambium 

 69) and the injured region covered with wax to prevent 

 drying, a thickened band of bark will slowly develop above 

 the girdle, but never below. Evidently the tissue above the 

 girdle obtains more food for growth, and this must come down 

 the stem in the sieve-tubes of the bark, for the wood continues 

 to have the upward current of water from the soil. A plant 

 so treated will die after some months, for the parts of the 

 stem below the girdle and also the roots cannot get the 

 necessary foods from the leaves. 



Another example : Trees often die because an iron wire 

 has been left wrapped around them until the stem has grown 

 so large that the ring of wire becomes embedded in the bark 

 down to the cambium, thus cutting off the sieve-tubes of the 

 bark and preventing foods from reaching the roots. Farmers 

 often girdle trees in early summer in order to prevent them 

 from shading crops; on some kinds of trees the leaves will 

 remain green all summer and sometimes die next year if 

 only the bark is girdled. But even the hardiest trees, like 

 the honey-locust, will fail to put forth leaves after two or 

 three summers, because the supply of food, which came down 

 from the leaves and was stored in the roots before the girdling, 

 is gradually used up, and no more can get down to the roots. 

 In order to make the leaves of such trees wilt soon after 

 girdling, it is necessary to chop through the bark and several 

 inches into the wood. Anticipating the lessons on stems in 

 Chapter VIII, it may be stated here that only the wood-tubes 

 in the light-colored outer layers of wood, known to lumber- 

 men as sap-wood, are important in the ascent of water ; and 

 therefore cutting deep into the sap-wood of a tree will cut 

 so many wood-tubes that the leaves cannot get water to 

 supply loss by evaporation, and hence soon wilt. 



Still another interesting proof that the prepared foods 

 from the leaves go down the stem in the bark next to the 



