AXATOhIY O F GRAFT U S I O N S 63
same reason there are never any connections between tissues i n the old wood a n d the healing masses, which are moreover covered by a n in- sulating cork tissue themselves.
64 INGEGERD DORMLING
The term intermediary tissue (or "Zn-ischengewebe"), 1 1 0 ~ ever, \voulti indicate both its place of formation a n d its inorphological a n d physio- logical importance-it is a true intermediary stage before subsequent differentiation.
I n the preceding account of the process of union I have intentionally avoided the word intermediary tissue or any other term that would distinguish the new formalion at the wound surfaces of grafts from those contiguous to other wound surfaces. On the contrary, there seem to be no fundamental differences. It has appeared that all cells, irrespective of their origin, have the power to effect union of graft components, a n d to obtain union with cells in the other graft components mhatever their origin m a p be. Vascular cambia as well as cork cambia m a y develop in tissues of any origin. It is true, certainly, that new formations between two graft components m a y assume different appearance to that of the callus at a n open wound surface. T h e callus tissues in grafts have a relatirely protected position against excessive drying. More tissues than those which usually profilerate in a n open wound participate in the callus formation, e.g. the wood parenchyma a n d superficial cells in the cortex. T h e new cells are often large a n d have thin walls. Dif'ferentia- tion occurs in callus tissues independent of their position-in the junction of a graft or in a n open wound.
K R E U I ~ E states (loc. cif., p. 357) that union of two graft components can occur not only by means of the intermediary tissue mentioned but also by means of true callus in the periphery of the graft wounds. This statement was apparently not founded on any of his own observations.
KRESRE worked only with herbaceous plants, a n d h e was consecpently only rarely confronted with phellogen formations in the cortex. It is probable, therefore, that the formation he thought to be "true callus in the periphery of the graft wounds" was actually callus produced by the cortex, which finally brings about a healing of a phellogen type.
HEKSE, too, stated that no distinction could be m a d e between callus a n d what h e called \\-ound cork. I myself would prefer to define a callus formation as a "wound cork" or a "wound periderm" when the divisions in it have assumed a regular pattern. phellogen h a s thus developed, a n d suberized products have been deposited externally.
Tissues between exposed wood surfaces did not occur i n KRENKE'S herbaceous graft material. These tissues are originally of the same character as all other newly formed tissues at the wound surfaces. It is only the conditions for their further development that change when the cambia close u p on the outside. They mill then remain as realinter- mediary tissues such as observed by GOPPERT.
AKATOMY O F GRAFT UNIONS 65
KRENRE defined callus as follo\vs: "Kallus ist jede durch IYundrei- zung hervorgerufene interorgane Bildung, die durch Wachstum und Zellteilungen enstanden ist". The same author also considered all living cells capable of participation in the formation of callus. The latter has been found true also in respect of pine. Referring to this statement and to what has been said above, it must be unrealistic to make a definite distinction between tissues formed at graft wound surfaces and those found at open wounds.
Naturally there is nothing to prevent tissues developed from the wound surfaces of both components in a graft zone from being called inter- mediary tissue, as long as one realizes that the formations are nothing but an ordinary wound-healing process going on from both sides under particularly favourable conditions, and that the intermediary tissue could just as well also be called callus. The concept of intermediary tissue should, however, in my opinion be limited only to such tissues as in one way or another are formed between two exposed wood sur- faces ( c f . the next chapter).
The term callus is, no doubt, somewhat diffuse, and is used to indicate wound-healing tissues of various kinds and at various stages of development. KREXKE used a classification of callus tissues (loc. cit., p. 234). According to this classification a callus is called primary until a cambium is formed within it. When the callus cambium has developed derivatives, the callus is called secondary. In the primary callus an immediate cell transformation may occur, e.g. by suberization and lignification. It is then reasonable lo call it a transformed primary callus.
If no regular cambium is developed in the callus, and irregular meristem- atic elements which continue to transform are established, K R E N I ~ E calls this differentiated primary callus. KKENKE himself considers this classification directly applicable to "intermediary tissue".
The first formations at the wound surfaces of grafts should un- doubtedly be called primary callus.\Yhen cambia start to develop within these formations, they advance towards the secondary stage. In rapidly healing grafts one finds nothing but primary callus, which first produces the parenchyma union and then the cambial union. As soon as the cambial junction has been achieved one can no longer speak of callus tissue in this context. The formations which heal the cut stock must be regarded as entirely secondary. In accordance with SORAUER, it would be better to speak of healing edges ("~berwallungsrander"). When primary callus formations at the cut surfaces of the two components do not unite with each other very rapidly, the divisions some distance inside the surface of this tissue assume a phellogen character. The
6 6 INGEGERD DORMLING
cambium from the unwounded part advances into the callus, which then passes into the secondary stage. T h e callus cells on the outside of the phellogen are suberized, a n d may be called "transformed primary callus". In the tissues enclosed between the wood surfaces of the graft components, immediate cell transformations (transformed primary callus) occur, as well as irregular meristematic elements (diferentiafed primary callus). Also secondary callus is sometimes present: vascular nodules may be formed and cambia which have first turned in between the wood surfaces may be cut off by the fusion of cambia outside the wood surfaces (cf. nest chapter).
D. Tissues between the wood surfaces of the graft components