junction between root stock and scion Wood. Often evident by a thickening of the trunk at the union.

graft union (Wikipedia)

Grafting or graftage is a horticultural technique whereby tissues of plants are joined so as to continue their growth together. The upper part of the combined plant is called the scion (/ˈsən/) while the lower part is called the rootstock. The success of this joining requires that the vascular tissues grow together. The natural equivalent of this process is inosculation. The technique is most commonly used in asexual propagation of commercially grown plants for the horticultural and agricultural trades. Grafting is the act of joining two plants together. The upper part of the graft (the scion) becomes the top of the plant, the lower portion (the understock) becomes the root system or part of the trunk. The scion is typically joined to the rootstock at the soil line; however, top work grafting may occur far above this line, leaving an understock consisting of the lower part of the trunk and the root system. Although grafting usually refers to joining only two plants, it may be a combination of several. In most cases, the stock or rootstock is selected for its roots and the scion is selected for its stems, leaves, flowers, or fruits. The scion contains the desired genes to be duplicated in future production by the grafted plant.

Cherry tree, consolidated "V" graft
Tape has been used to bind the rootstock and scion at the graft, and tar to protect the scion from desiccation.
A grafted tree showing two differently coloured blossoms

In stem grafting, a common grafting method, a shoot of a selected, desired plant cultivar is grafted onto the stock of another type. In another common form called bud grafting, a dormant side bud is grafted onto the stem of another stock plant, and when it has inosculated successfully, it is encouraged to grow by pruning off the stem of the stock plant just above the newly grafted bud.

For successful grafting to take place, the vascular cambium tissues of the stock and scion plants must be placed in contact with each other. Both tissues must be kept alive until the graft has "taken", usually a period of a few weeks. Successful grafting only requires that a vascular connection take place between the grafted tissues. Research conducted in Arabidopsis thaliana hypocotyls has shown that the connection of phloem takes place after three days of initial grafting, whereas the connection of xylem can take up to seven days. Joints formed by grafting are not as strong as naturally formed joints, so a physical weak point often still occurs at the graft because only the newly formed tissues inosculate with each other. The existing structural tissue (or wood) of the stock plant does not fuse.

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