165 



PART V.-WOUND3 AND DISEASES. 



CHAPTER I. WOUNDS. 



143. If a mass of living cells is cut Healing of 



through with a sharp knife, the exposed surface at first turns ^JJJjfjf 1 ^ 

 brown and then becomes covered with a protecting coat of Cork Layer, 

 corky tissue. The cells actually cut through die at once, and in 

 consequence of the access of air some of their contents become 

 oxidised, this process causing the brown discolouration. The 



layer of cork is formed by the uninjured cells immediately 

 below the cut surface which, by their growth and division, 

 give rise to layers of flat tabular cells. These soon die and, 

 their cell walls having become converted into cork, form a pro- 

 tective coat several layers of cells in thickness which, although 

 elastic, is very impervious to air and water, and consequently 

 the rapid drying up and destruction of the .living cells beneath 

 it is effectually prevented. This process may be well observed 

 on the cut surface of a potato, although the wounded surface 

 of any mass of living cells, in the root, stem, leaf, or elsewhere, 

 will present essentially the same phenomenon. 



144. If the cells below the cut Callus- 

 surf ace are growing and dividing when the cut is made, the first ^^^lin 

 thing which happens is the dying of the actually cut cells and of Wounds 

 then the external corky coat is formed as before. Now, this by Ooclu- 

 thin layer of cork being very elastic, the pressure exerted by it Slon> 



on the living growing tissues beneath it is very much less than 

 was caused by the original external tissues which have been re- 

 moved by the cut. These living tissues are consequently 

 able to grow far more vigorously than before and they give 

 rise to a juicy cushion of thin-walled cells which is called a 

 callus. This at first consists of a mass of embryonic cells ; 

 some of these remain capable of growing and forming new 

 cells, thus retaining their character as embryonic tissue, while 

 the remainder become gradually differentiated, very much as 

 is the case in normally growing tissues. This process may be 

 well seen on the vigorously growing stem of a young sapling. 

 Suppose, for instance, that we cut away a longitudinal strip 

 of the external tissues from such a stem, the cut extending 

 down to the wood which is thus left exposed. The wound at 

 first gapes owing to the contraction of the tissues of the cor- 



