174 APPLIED BIOLOGY 



Wood-tubes and Sieve-tubes. The tubes in the wood con- 

 duct the materials collected by the roots up to the leaves; 

 and the sieve-tubes of the inner bark distribute the food 

 materials prepared by the leaves to various parts of the 

 plant body below the leaves. 



Epidermis and Corky Layer of the Bark. These cells are 

 protective in function. They prevent mechanical injuries 

 to the delicate cells beneath, and they protect against loss 

 of water by evaporation, and against sudden changes of tem- 

 perature. 



Parenchyma or Pith of the Bark. In young stems this 

 contains chlorophyll and is able to manufacture starch, but 

 in older stems it serves merely as a storehouse for foods. 



Cambium. The region in which all the new cells are 

 formed for secondary growth in diameter of stems. 



Medullary Rays or Pith-rays. The paths along which 

 liquids may pass from the wood outward across the stem to 

 the cambium and bark, and inward from the bark to the 

 cambium and cells of the wood. The cells of the rays are also 

 important for the storage of food. Some stems (oak, willow, 

 hazel, and lilac), if collected during the winter and preserved 

 in alcohol, will show, if thin sections are made and treated 

 with iodine, that starch is stored in the rays and in the pith 

 of the bark. In linden and birch stems the presence of fat 

 can be demonstrated in sections by the use of alcana-root 

 dissolved in 70 per cent alcohol, which stains fat red. In 

 still other trees sugar is stored in the pith-rays. 



166. The Cambium in Grafting and Healing Wounds. 

 The power of growth of the cambium is well illustrated in 

 these two processes. If two branches of a tree be scraped 

 down to the cambium and the wounded surfaces brought to- 

 gether, the rapid growth of cambium cells will cause a grafting 

 or union of the two. Examples of this can often be seen in 

 a forest or orchard, for branches often grow so near together 

 that the swaying by the wind wears away the bark, exposes 



