CIRCULATION AND USES OF FOOD BY PLANTS 99 



two potatoes of equal weight are balanced on the scales, the skin 

 having been peeled from one, the peeled potato will be found to 

 lose weight rapidly. This is due to loss of water, which is held in 

 by the skin of the unpeeled potato (see right hand figure below). 



There are also small breathing holes known as lenticels scattered 

 through the surface of the bark. These can easily be seen in a 

 young woody stem of apple, beech, or horse-chestnut. 



Experiment to show that the skin of the potato (a stem) retards evaporation. 



Proof that Food passes down the Stem, — If freshly cut willow 

 twigs' are placed in water, roots soon begin to develop from that 

 part of the stem which is under water. If now the stem is girdled 

 by removing the bark in a ring just above where the roots are 

 growing, the latter will eventually die, and new roots ^^dll appear 

 above the girdled area. The food material necessary for the out- 

 growth of roots evidently comes from above, and the passage of 

 food materials takes place in a downward direction just outside 

 the wood in the layer of bark which contains the bast fibers and 

 sieve tubes. This experiment with the willow explains why it is 

 that trees die when girdled so as to cut the sieve tubes of the inner 

 bark. The food supply is cut off from the protoplasm of the cells 

 in the part of the tree below the cut area. Many of the canoe 

 birches of our Adirondack forest are thus killed, girdled by thought- 



