THE GROWING PLANT 55 



through the bark and a little into the wood is cut in 

 the stem of any of our common woody plants during 

 spring or summer, a callus or cushion of new cells (72) 

 will soon form on the upper side of the notch, but not 

 on the lower, showing that the material from which new 

 cells are formed is passing downward. Close examina- 

 tion will show that this callus forms just outside the 

 union of the bark and wood. In all plants having more 

 than one cotyledon (45), this current is through the 

 sieve tubes in the inner layers of the bark. The pre- 

 pared food matter dissolved in water passes readily through 

 the cell-walls, and passes from the leaves to other parts 

 of the plant by diffusion (63). 



80. Killing trees by girdling. To destroy the life of 

 a tree that cannot be conveniently removed, we girdle 

 it by cutting a notch about the trunk beneath the lowest 

 branch. This cuts off the downward food current and 

 so starves the protoplasm of the roots. If the notch is 

 made after the leaves have expanded in spring, and ex- 

 tends only through the bark, the leaves may remain 

 fresh for several weeks, for the transpiration current 

 passing through the sap-wood (77) may continue. Since 

 the roots receive no nourishment, however, they will 

 soon cease to grow and will usually die from starvation 

 before the following spring. If the notch is cut deep 

 enough to reach through the sap-wood, thus cutting off 

 both the ascending and descending currents, death of the 

 tree follows soon. 



81. Root starvation may occur without girdling. In 

 seasons of extreme drought, when the leaves are poorly 

 supplied with crude food materials from the soil, the 

 amount of prepared food may be so meager that the food 

 current will be exhausted before it reaches the roots. 



