873 



ing mesophyll. These form a cuticle and thus represent a complete cover- 

 ing of the wounded leaf surface. When this covering is complete, the dead 

 tissue is thrown off. In this the pressure of moist air is taken for granted. 

 In other cases a normal periderm is formed from several cell layers which 

 suffices as a protection for the healthy leaf tissue. 



The second case of the healing of leaf injuries, viz : by callus formation, 

 is explained by the accompanying figure. It is the cut wound from a cut 

 on Leucojum vernum. The wound lay in the open space between the two 

 tissues of lamellae / and f •,vv vv are the edges of the wound with the dead 

 pieces of tissue. The wound cavity is now filled by the callus cells devel- 

 oping by elongation from the fresh tissue, which lack chlorophyll and have 

 suberized walls. The normal condition of the leaf is represented at the 

 left side of the figure where i i indicates a large air chamber; the tissue sur- 

 rounding it has not been changed by wound stimulus, o is the upper and u 

 the .under side of the leaf. Many fleshy leaves react according to this 

 scheme, but their processes of healing vary greatly, depending on the subse- 

 quent participation of the process of cork formation. Complete union of 

 the edges of the wound can also take place, as may be observed, for example, 

 in the cut surfaces of fleshy roots and tubers^ The union is sometimes the 

 result of organic coalescence, sometimes only a cementing of the surfaces 

 since the cut cells are changed into a gum-like mass by the swelling and 

 disintegration of their walls. 



The leaf can under certain circumstances reproduce the part arti- 

 ficially removed (regeneration, according to Kiister) or form a compen- 

 sating organ (restitution") according to the specific, character of the leaf, 

 its youth and its distance from the reserve-substance containers. 



Frequently whole leaves, or pieces of leaves, removed from the plant, 

 can form new roots and aerial axes. This capacity is utilized for 



Leaf Cuttings. 



The best known and most frequent use of leaf propagation is found 

 in begonia culture. According to Hansen^, in the various varieties of 

 Begonia Rex wounds produced by slashing the nerves of the leaf lying flat 

 on the soil are closed at once by callus. In this way a tuberous tissue is 

 formed on the mother leaf from which tissue, or that immediately sur- 

 rounding it, roots develop ; later, sprouts are formed from the same tissue, 

 which, however, do not develop their own roots but are nourished by the 

 above-mentioned roots of the callus. Sprouts develop there from one or a 

 few cells of the epidermis near the cut rib, sometimes nearer, sometimes 

 farther from the wound. In such cells, a horizontal partition wall is pro- 



1 Fig-dor, Wilhelm, Studien iiber die Erscheinung- der Verwachsung- im Planzen- 

 reiche. Sitzungsber. d. Akad. d. Wissensch. Wien; cit. Bot. Zeit. 1S91, No. 23. 



2 Figdor, "Wilhelm, tjber Regeneration der Blatt-spreite von Scolopendrium. 

 Bericht d. Deutsch. Bot. Ges. 1906, Vol. XXIV, Part 1.— Figdor, Wilhelm, tJber Resti- 

 tutionserscheinungen an Blattern von Gesneriaceen. Jahrb. f. wiss. Bot. 1907, Vol. 

 XLIV, Part 1. 



3 Hansen, Ad., Vorlauflge Mitteilung. Flora 1879, p. 254. 



