GROWTH AND REGENERATION 



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cells are derived from other cells. Or, when a bone is broken, 

 the ends of the bone will knit by the formation of new 

 cells and the deposit of bone material about these new cells. 

 This healing of skin or bone or other tissues is widespread 

 among all kinds of animals and plants, and may be considered 

 as a growth in response to stimulation set up by injury. 



Fig. 105. Growth in a thread-shaped body 



A body that grows by increasing in length only, as do thread-shaped algae, like the 

 Spirogyra, and as do the threads of fungi, changes the ratio of its volume to its surface 

 very little. In the diagram a cube is represented as increasing in one direction to seven 

 times its original diameter. With this growth the area has increased to five times the 

 original surface ; and with each addition in length the discrepancy becomes less and less 



Not all kinds of tissues will produce new cells of the same kind. 

 For example, we learned that the number of nerve cells in the body 

 does not seem to increase after a child is bom. An injury to the 

 brain will heal by the formation of a scar consisting not of neurons 

 but of connective tissue. In the same way many kinds of wounds 

 leave scars of connective tissue that close the gaps and hold the parts 

 together but do not function in the same way as the specific kind of 

 cells that were destroyed by the wound. On trees we often find scars 

 consisting of callus, produced as the result of some mechanical injury. 



