382 GENERAL PLANT PATHOLOGY 



wound-wood is produced near the place of injury, the transitional forms 

 at a greater distance and then the long-celled zone, which is formed 

 from undivided cells of the cambium. The daughter cells of the cam- 

 bium of the short-celled zone form near the edges of the injured part, 

 a wound-wood composed of polyhedric fundamental tissue cells re- 

 sembling the medullary ray cells of normal wood, only a few of such ele- 

 ments develop into parenchymatous tracheids. The cells of the long- 

 celled zone retain the character of wood parenchyma, but between them 

 narrow vascular cells united into strand-like groups are formed, while 

 wood fibers and broad ducts are absent. Such formed elements have 

 been termed primary wound-wood by de Vries, and later, there occurs 

 the production of a secondary wound-wood in which the cells gradually 

 assume a normal form. Abnormal resin ducts are formed in wound- 

 wood and these ducts are often more numerous in abnormal wood than 

 in the normal. 



Sometimes the wound-wood does not form definite stratified tissues. 

 Occasionally tracheid-like cells are found in the callus which become 

 united into ball-like groups separated from the normal wood. Wood 

 fibers, which have an irregular course, have formed the gnarled wood. 



The pith may take part in the formation of wound-wood, for it is 

 highly capable of producing callus, and also from the ground tissue of 

 injured leaves. No definite outer form is characteristic of wound-wood. 

 Frost action may kill the cambium in places, and if the dead places are 

 surrounded by cushions of wound-wood, then we speak of frost canker. 

 Frost cracks are filled with wound-wood, which close up the wound 

 followed by the formation of a frost ridge. Such canker tissue may be 

 destroyed during a frosty spell and a new attempt to form Callus results 

 in the addition of new wound-wood to the old and frost cankers are 

 formed. 



Sometimes without an injury, tissues resembling wound-wood are 

 formed by the activity of the normal cambium, or from a newly formed 

 independent cambium. Under some conditions, the parenchyma of 

 the medullary rays increases at the expense of the formed elements of 

 the wood, so that broadened medullary rays are formed. Fasciated 

 branches frequently show such broadened medullary rays. Tuber-like 

 gnarls are formed in fruit trees that have stone fruits, and also in 

 beech bark and the structure of gnarls has been investigated by 

 Sorauer, and the bark tubers of beech by Krick. 



