The movement of water in cell-structure, transpiration in the leaf, and the downward current of assimilated food belongs to botany. But the work of the horticulturist soon shows the need of some knowledge relative to sap-movement. This is specially true of the descending movement of assimilated food. As an example, if a label wire is left on the stem of a young tree we find that it soon is sunk into the bark and wood, and the part above enlarges, while the stem below seems to grow smaller, as shown in Fig. 11.
Fig. 11. - Stem of young tree girdled by wire. (After Bailey.)
Aside from scientific theory this would clearly show that cell-forming material descends near the cambium layer. In the same line, if we remove a ring of bark in June from the stem of an apple-tree we obstruct the downward movement under the bark. Soon we begin to observe a process of healing over the wound by cell-formation on the cut surface above the ringing, and as the season advances we notice that the ripened and mature condition of the wood above has changed leaf-buds into fruit-buds. Again in spring budding, those with most experience make the cross-cut of the T below, and shove the bud upward as the cross-cut above obstructs the movement of the assimilated food from the leaf.
In regard to sap-pressure we have also a good example in working scions on uncongenial stocks. As instances, the apricot on myrobalan stocks makes no real union of cells. The cells of the apricot rest on cells of the myrobalan. The growth seems to be thrifty and unimpeded, but the winds usually topple them over before they reach the bearing-stage. When blown over it is found that there is no rupture of cells, but a clean separation of the two woods. Where the stock and scion are so nearly allied that the union of cells takes place, breaking down causes rupturing of the wood. Yet the wood of the stock and scion remain distinct. If orange is budded or grafted on lemon the shoot that starts from the stock half an inch or less below the union will be lemon. But what are known as "graft hybrids" have come from sprouts from the point of union.
But this is rare, perhaps for the reason that shoots rarely start from the exact point of union of stock and scion.
In Fig. 12 the stock has grown faster than the scion yet the union of the cells of stock and scion can be plainly seen.
Fig. 12. — An old cleft graft, showing point of union of stock and scion.