STEM GRAFTING 



257 



In the higher animals, including man, whole organs are not 

 restored. A process similar to regeneration is evidenced, though, 

 in the healing of a wound and in the knitting of a broken bone. 

 An ordinary scratch or even an extensive cut often heals without 

 so much as a scar, because the destroyed cells are quickly re- 

 placed by new cells. The capacity of lower tmimals to regener- 

 ate an entire individual from a portion of another individual is an 

 extreme example of the same ability as is manifested in the heal- 

 ing of a wound. The type of regeneration shown in the regrowth 

 of destroyed tissues is known as physiological regeneration in con- 

 trast with the regeneration that is a method of propagation. 



Grafting. If, under certain conditions, the freshly cut surfaces 

 of two plants are brought into contact with each other, the plants 

 will grow together as one. People for many ages have been using 

 this knowledge for the propagation of fruit trees. A stem called 

 a scion is cut from a tree and is so attached to a rooted stem called 

 the stock that the cambium or growing layers of each are brought 

 into close contact. The actively growing cambium cells of each 

 unite the two stems. In time, food from the stock will pass 

 through the ducts and nourish the scion. This is called a stem 



^1 **• »^ 



Planarid 



Animals often regenerate lost parts. The lower animals, when injured, may grow back 

 few or many lost organs. The simpler the animal, the greater is the power of regeneration. In 

 the human body only limited regeneration takes place. 



