455 



The upper figure in illustration 90 shows the process of healing. It is 

 a cross-section through the branch with a depressed wound, which may be 

 seen at the base of the picture, showing the habit of growth. M is the pith 

 with its sHme cells ; H, normal old wood ; R, bark. It is seen at the wound 

 that the tissue atrophy originally included the whole bark {R). The wood 

 cylinder (//), however, was not attacked. The edges of the bark wounds 

 {ivr) died and were separated by a full cork layer {t) from the healthy 

 bark parenchyma at the sides. In the remaining part of the bark, a new 

 growth in thickness had set in, which manifested itself by forming the 

 primordia of new hard bast strands {b'). The old hard bast near the wound 

 was diseased and found shut in by a cork envelope. 



The whole tissue zone (Z^'-&')had been formed anew subsequently, and 

 indeed in those parts covered by the bark by means of a normal cambial 

 activity, but at the wound itself by an increase of the youngest sapwood. 

 For the wound destroyed the cambium, and accordingly the last formed 

 cambial wood layer has started a renewed increase of cells and has formed 

 callus-like tissue. The primordia of the vessels, which at the time of the 

 deposition of the latest sap-wood had already become thick-walled, have, 

 however, not taken part in the increase, but have been pushed outward 

 passively by the newly formed callus. It is seen in this, that these primordia 

 of the vessels {g'), which in the cross-section resemble the ducts {g) in the 

 normal wood {H), now occur isolated in the callus tissue. 



The healing process becomes more exactly recognizable in the lower 

 anatomical figure which represents a piece of tissue from around the hole 

 in the upper cross-section. H again represents the old wood with some 

 vessels {g). Where the elements, represented with thick walls, cease, is 

 seen the most depressed part of the wound. On this remain the youngest 

 elements of the sap wood, which had increased in size and number after 

 the phenomena of decay had ceased. The immature sap wood, already 

 differentiated, became imore porous and thin walled, and thus it happens 

 that thin-walled vessels ig') may be found again in a delicate parenchyma 

 wood. All the tissue indicated by (n) has been newly formed, its produc- 

 tion corresponding with the new formation of bark on peeled trunks. The 

 new tissue, developed from the callus, already exhibits some differentiation. 

 This differentiation indicates that the stem is about to form new bark where 

 it was injured, for in the region directly in front of the thin-walled vessels 

 {g'), we find the first parallel cell divisions indicating the formation of a 

 new cambial zone. Besides these, the primordia of secondary hard bast {h') 

 may be recognized, to be sure, even in parenchymatous tissue with a plastic 

 content but not containing chloroplasts, which later becomes normal bark. 



This healing process, however, has only been observed when the plant 

 had direct sunshine and fresh air in circulation. I have learned to recognize 

 the disease as occurring in greenhouses and indeed in those where because 

 they contain plants from warmer climates the air is enclosed and very moist. 

 In one special case, the abundant ventilation in the greenhouse stopped the 



