150 MINUTE STRUCTURE OF THE STEM. 



the wound exhales moisture very rapidly, and under ordinaiy 

 circumstances, except in spring, soon becomes dry. As Hartig 1 

 has shown, the drying -of the exposed tissues is fatal to their 

 component cells, and the organic contents speedily undergo 

 chemical decomposition. The products of this decomposition 

 have been further shown by him to be fatal to neighboring cells, 

 and under certain conditions the mischief may progress to an 

 irreparable extent. But usually there is an arrest of the de- 

 structive action either from lack of the free oxygen necessary for 



/ Cs *s 



the putrefactive process, or by the protection afforded by tissues 

 for repair. Wounds in resinous trees are measurably hindered 

 from effecting much damage, owing to the exudation of liquid 

 resins which exclude air. 



421. The smaller wounds of a plant are generally healed by 

 cork or by callus. 1. By cork. The superficial layer of cells at 

 the surface of the wound is destroyed by the injury, and dries 

 at once. In soft tissues the layer just below this immediately 

 becomes merismatic, and behaves precisely like normal cork- 

 meristem, covering the entire wound with a grayish or brownish 

 film, which is in unbroken connection with the edges of the 

 wound. Extreme dry ness of the air, or, on the other hand, ex- 

 treme humidity, hinders repair by cork. 2. By callus. This is 

 best studied in leaves and in ' cuttings." When a young, juicy 

 leaf is wounded by an incision, some of the cells at the exposed 

 surface may give rise to elongated sac-like bodies, which fill up 

 the greater part of the injured cavity, and, according to Frank,' 2 

 serve as a new epidermis. Or small cells in close apposition 

 may be at once formed, and completely protect the tissue below. 

 In " cuttings" the callus immediately forms a swelling near the 

 wound. A portion of the callus may by continued cell-division 

 extend over the cut end, everywhere bounded on its exposed 

 surface by a cork layer. Activity of the cells in the callus and 

 around the fibre-vascular bundles soon gives rise to new parts, 

 for instance, roots. 



422. It often happens under favorable conditions that a large 

 mass 3 of tissue is gradually formed around, and finally over, a 

 large injured surface. 



1 Zersetzungserscheinungen des Holzes, Berlin, 1878. (Quoted by Frank.) 



2 Die Pflanzenkrankheiten, 1879. 



3 Usually when a branch dies it remains attached for a while to the stem ; 

 and no wound is in fact caused until the slow desiccation of the deeper tissues 

 has gone on to a considerable extent, and without exposure to atmospheric 

 air or outside moisture. "When the branch at last falls off, the tissues around 



