STEM STRUCTURE 79 



Pith. The pith may be a minute remnant of the formative tis- 

 sue, or a larger storage place for foods and the pith rays serve as 

 cross channel for liquids to follow in their circulation in the stem. 



So we have one protective region, the epidermis; one digestive 

 region, the cortex; one formative region, the cambium; one storage 

 region, the pith. The ducts, soft bast, and pith rays are the chan- 

 nels for circulation of fluids while the wood and bast fibers are for 

 strength and support. 



Grafting. The remarkable ability of the cambium cells to grow 

 and produce new tissues is utilized in grafting. Grafting consists 

 in bringing into close contact the cambium layer of a small active 

 twig with that of the tree upon which it is to grow. This may be 

 done by splitting the stem, and inserting the fresh-cut twig, or by 

 raising the bark and inserting an active budded twig beneath it, 

 with the cambium layers in contact. The wound is then protected 

 by wax and growth between the two cambium layers soon unites 

 the new stem with the old. 



The cambium also provides for the healing of injuries and the 

 covering of scars where branches are cut off. New tissue forms at 

 the edges of the wound and gradually covers the whole area, pro- 

 vided that spores and bacteria do not first cause decay of the ex- 

 posed surface. To prevent this, cut or injured surfaces should al- 

 ways be tarred or painted to kill and keep out bacteria, while new 

 tissue is growing. If decay has begun the rotted wood must be 

 cleanly removed, the cavity sterilized with tar and filled with 

 cement. The cambium growth will now extend the tissue inward 

 from the edges and often cover the scar, filling and all. , 



In rare instances two limbs, or even two separate trees of the 

 same kind, will chafe together in the wind, till the cambium is 

 exposed in both. Then if undisturbed, an automatic graft may 

 occur and a curious condition will develop, in which the two trees 

 will continue to grow firmly together. 



Other Kinds of Stem Structure. In the chapter on seed struc- 

 ture it was stated that plants whose seeds had two cotyledons 

 (dicotyledonous plants) had stems that differed from plants whose 

 seeds had one cotyledon (monocotyledonous) . The stem just 



