PATHOLOGIC PLANT ANATOMY 



377 



the axillary fibrovascular cord is resolved into a circle of isolated bundles 

 separated by chlorophyll-containing cells. 



2. Callus 



Callus may be defined in the widest sense of the word as all cell and 

 tissue forms produced subsequent to and as a result of injury. In 

 many plants and plant organs, only a metaplastic change of the cells 

 was incited by the injury (callus-metaplasia); in others, the cells laid 

 bare showed an abnormal growth and were changed into voluminous 

 vesicles and sacs (callus-hypertrophy), or an 

 increase of the normal tissue may result from 

 wound stimuli (callus-homooplasia). The 

 cells may be abundant after an injury owing 

 to active cell division and heteroplastic tissue 

 arises (callus-heteroplasia). When excres- 

 cences arise, which are composed of cells very 

 little differentiated and of the simplest form, 

 they are called cataplasms. If produced after 

 injury, they are found to differ greatly. The 

 tissues produced after an injury, if resembling 

 cork, are termed wound-cork, if similar to those 

 of wood, they are called wound-wood and 

 where we have the healing tissue composed of 

 nearly homogeneous parenchyma, it is called 

 simply callus. 



Callous tissue may be formed as wound 

 tissue in very different plant groups. It has 

 been found in the algal fungi and vascular 

 cryptogams. The woody seed plants have been studied carefully as 

 to the formation of callus, because of its economic importance in forestry 

 and horticulture. Rose, poplar, or willow cuttings kept in moist air 

 and at a proper temperature after a few days form a ring-like tissue 

 excrescence from the cambium of the cut surface. This spreads out 

 rapidly and finally closes over the wound. Such rolls of tissue have 

 been called callus (callus, hard skin). 



Callus at least in its first stages appears in the form of a ring, some- 

 times it is irregular in its formation, often being lacking in some places 



Fig. 150. — Longitudinal 

 section of a callused end of 

 a cutting. C, C, Callus de- 

 veloped from cambium; H, 

 wood; R, bark. (After 

 Kiisler, p. 159.) 



