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
ANATOMY O F GRAFT U N I O S S 67
on both sides, the intermediary tissue m a y retain parenchynlatous con- nections with the tissues on the outside through rays in the newly formed wood.
Most of the cells in the intermediary tissue do not retain their strictly undifferentiated form for any appreciable length of time. The surface cells are suberized, as mentioned above; many others soon lose their living contents, a n d their walls lignify, but do not assume a tracheid character. Others which remain alive are incrusted with tannin, but a few i n common with the ray cells, may long retain a n unchanged character. It also happens that callus cells assume a more or less tracheidal appearance although they have no contact with any meri- stematic zone. The cells which a r e seated far out i n contact with the rays of the new wood, stay alive longest. Plate IX: 7 shows intermediary tissue from a three-year-old graft. Most of the cells are dead, but a few, mainly at the outer margins, are still alive.
Not infrequently cambial strands occur in the intermediary tissues without having any connection with the cambium on the outside.
Regions in which the meristematic activity continues m a y in cross- sections assume the appearance of islets surrounded b y dying a n d tannin containing tissues. A region of this kind m a y be seen i n the middle of the intermediary tissue in Plate IX: 4. Sections immediately above a n d b e l o v that in Plate I S : 4 show several cell divisions in the region. Plate I S : 5 and G are obtained from the same graft. The section i n Plate IX: G is approximately 13 m m above that shown i n Plate IX: 4, a n d the section i n Plate IX: 5 is situated between the two. I n the inter- mediary tissue in Plate IX: 5 only a few cells with living contents are left, mainly at the edges, a n d i n contact with some rays at the wood surfaces. The section in Plate I S : 6 is passing through another meri- stematic centre i n the intermediary tissue, which centre is not connected with that shown i n Plate IS: 4. A number of tracheids have been differentiated.
Several meristematic centres m a y occur close to each olher in the intermediary tissue (cf. Plate IX: 8). The cells in the nodule-like forma- tions are extended a n d bent, but entirely closed circles are rarely seen.
A remarkable fact about these structures is that the xylem is always seated on the outside of the cambium, a n d the phloem inside. Dif'feren- tiation of xylem elements continues as long as space is available, but the formation of new tissues on the phloem side gradually decreases, a n d the existing sieve cells are compressed.
These formations i n the intermediary tissues cannot be regarded as synonymous ~ v i t h the nodules ("sfaroblasts" according to LAGERBERG
68 IKGEGERD DORMLIXG
1943) that occasionally appear in the outer parts of the stem. In this case the meristem is established as a ring, but an entirely closed ring with phloem on its outside. hlore accurately it is a sphere, since the formation is round like a ball (cf. LAGERBERG). A broken leaf or branch trace may constitute the centre around which the formation starts. The nodule may continue its growth for a long time-as long as the surround- ing cells are able to supply water and nourishment. Plate IX: 10 shows a section through the centre of a nodule obtained from a one-year-old veneer graft. The nodule occurred in the uppermost part of the stock flap, which was shaped as in Fig. 17b, and was completely united with the scion. hleasurements have shorn-n it to be nearly spherical. This structure, as well as the formations in the intermediary tissues men- tioned earlier, all lack connection with the cambium of the stem or its vascular elements. They are consequently entirely dependent on the surrounding parenchyma for their supply of water and nutrients.
Cambia between the wood surfaces of the graft components may become isolated by the cambium of one component turning into the space between the wood surfaces and subsequently establishing junction with the cambium of the counterpart-not with its end, however, which lies between the wood surfaces, but at a place further outwards. The cambium part thus cut off from outward connection continues to grow as far as space permits, and it may also spread do\s.nwards and up- wards in the intermediary tissue. Plate IX: 11 shows how a cambium emerging from the stock has been intercepted by the union of an outer part of this cambium with that of the scion. How large a part of the xylem in the intermediary tissue has developed after the union on the outside can be clearly distinguished. The position of the stock cambium at the time of union with the cambium of the scion is marked with arrows. The pith of the scion has been compressed to make room for the expanding tissues. Approximately one m m further down, the stele in the intermediary tissue is entirely isolated from the vascular tissues on the outside (Plate IX: 12). The cambium has developed an arch, with the small phloem part turned inwards.
No cambial strands h a r e yet been developed in the intermediary tissues of the 15-week-old graft shown in Plate VI: 10, but the division activity is vigorous in the three clearly separated callus formations. The cambium from the scion on the left side in the photograph has first started to expand inwards and then bends outwards in order to meet the advancing stock cambium. There has been no union at all lligher up on this side, and the entire cambium developing from the scion enters between the wood surfaces. \\'hen cambia of both the graft
ANATOMY O F G R A F T UNIONS 69
components have finally united, some cambial tissue will definitely remain inside and become isolated in the same way as described above.
In the two other parts of the intermediary tissue, cambia would probably have developed spontaneously.
Needless to say, one has also to consider the possibility that some of the spontaneously developed cambia in the intermediary tissue may establish union with the cambium of either component, even though no direct evidence to this effect has been found in this material. After such a junction has been established, it would not differ in appearance from a cambium turning inwards from the outside. There is certainly no fundamental difference between the cambia developed spontaneously in the intermediary tissue and those from the exposed cambial edges.
Thus the union between the cambia of two graft components through a callus mass need not necessarily be achieved solely by the advance- ment of the cambial edges; it can also be established by the participa- tion of cambia developed in the callus.
In side slit grafts the intermediary tissues often reach a great volume.
The callus originates mainly from the innermost corner and the outer part of the scion (point 4 according to Fig. 21), but parenchyma and less differentiated cambial derivatives on the wood surface ofthe stock, as well as wood parenchyma in the scion, may participate. The tissues continue to expand, thus forcing the scion away from the stock until a union between cambia from point 4 and the incision face is achieved (Plate XIII: 10). When the pith of the scion has been cut through in the part turned towards the mood side, its callus formation is often con- siderable, as is also that from the rays in the stock situated straight opposite.
The longevity of the intermediary tissues is highly variable, and depends on the completeness of the connections with the tissues on the outside. The cells seated at the extreme edges of the intermediary tissue and connected with rays in the wood in the union zone, should have the same possibility of continuing living as the cells of the pith, which are fairly long-lived in pine. The isolated cambia remain active as long as the space and the supply of water and nutrients permit. For how long a time after grafting living cells may be found has not been estab- lished, since the oldest grafts examined were only three years old.
E. Some observations on the shaping and fitting of the graft components