HEALING OF WOUNDS ON WOODY STEMS 131 



young apple and pear shoots. On ordinary shoots they are 

 developed at the places where stomata occur in the epidermis 

 and serve for the admission of air through the periderm into the 

 intercellular spaces of the medullary rays and other parts of the 

 stem. 



(/) Healing of wounds on woody stems. — Wounds made into 

 the soft parenchymatous parts of herbaceous stems, leaves, tubers, 



n 



Fig. 61. — A, Stem with amputated branch (ii) ; c callus. 



B, Longitudinal section oZ A\ c callus formed by exposed cambium ; b exposed wood 

 of the branch, 



C, Longitudinal section after the exposed wood of the branch has been completely 

 covered over by ^ve annual growths («). 



and fruits soon become healed over by the formation of a layer 

 of cork-cells which develop from the uninjured cells exposed 

 by the wound. When the mature wood of a stem or branch 

 is exposed {b, Fig. 61) it becomes covered by the gradual 

 extension of a tissue manufactured chiefly by the cambium. 

 The cambium exposed by the cut and the very young cells of 

 the wood and bast at first give rise to a mass of soft parenchyma- 



