PATHOLOGIC PLANT ANATOMY 379 



the cambium the primary and secondary bark tissues. The epidermis 

 plays an unimportant role. Pith also can develop callus. 



The investigations of R. Hoffman, Kiister and Stoll go to show that 

 the cambial cells when division takes place after injury are not re- 

 stricted to the mode of normal division but can grow in every direction. 

 It is certain, therefore, that the conditions of changed pressure are of 

 importance and significance, and yet this fact alone is hardly sufl&cient 

 to explain the phenomena of growth subsequent to an injury. The 

 cell divisions are very regular and rapid in those woody plants which 

 form callus. 



Cuttings of woody plants, such as Populus pyramidalis (Fig. 150), if 

 placed in water and covered with a bell glass, so that the upper end 

 extends above the water into the moist air, shows early division of the 

 cambial cells near the upper wounded surface. We find these cells 

 are divided by walls perpendicular to their long axis, and in a lively 

 manner, by forming tangential walls, causing an abnormally intensive 

 growth in thickness of the cutting near the injured place. A strong 

 callus has been formed by abundant division of the cambial cells and the 

 cutting assumes a club-shaped form at its upper end. The wedge, 

 which is formed in this way between the wood and bast, has been termed 

 by Th. Hartig the "Lohdenwedge," which might be termed more ap- 

 propriately in English the healing wedge. In the formation of this 

 wedge, the cambial cells have divided just as under normal conditions, 

 but the relief of pressure has caused some of the outer cells to protrude 

 to form the enlarged part of the wedge with the outer cells bent 

 strongly. Primary bark as in Salix easily forms callus, and petioles and 

 leaves often form abundant callus. 



Histologically the tissues of callus are distinguished by the slight 

 differentiation of their cells. The cushions of callus in many kinds of 

 cuttings are made up of the same kinds of cells and in a homogeneous 

 fashion. The cells are typically nascent ones with thin cell wall, pro- 

 toplasmic contents and a colorless cell sap. If the growth is slow, the 

 callous cells are small and closely fitted together, but with rapid growth 

 the cells are large and loosely placed with conspicuous intercellular 

 spaces. Tracheids are absent from the upper cells of the cushion of 

 callus, but in the lower part of the healing wedge some of the cells 

 assume the tracheal character. The formation of a tegumentary layer 

 is next to the development of tracheids the most interesting process of 



