Adventitious root formation in cuttings develops through several stages.
- formation of root initials
- subsequent development into root primordia
- emergence of roots through the epidermis of the stem
The first stage of root formation in most cuttings is called dedifferentiation or the induction phase. It is characterized by an internal change in the parenchyma cells in the stem that leads to initial cell divisions.
The concept of dedifferentiation is that cells with a previous fate are able to be induced to follow a different developmental pathway. In the case of root formation, cells that were stem cells now begin the process of becoming a root meristem.
The stages of root formation can be illustrated in cuttings from eastern redbud (Cercis canadensis). Cross sectional stem anatomy is illustrated above.
Dedifferentiation is a change that occurs in certain stem cells in response to wounding. It allows the change in the cells fate from a stem cell to a new root cell.
Certain cells associated with the vascular bundle (usually phloem parenchyma cells) divide and become meristematic.
Five days after taking the cuttings, the cells in the cortex just beyond the phloem but under the sclerenchyma ring become activated and divide.
Notice how some cells in the pith are dividing, but the orientation is random and these will not result in root formation.
The cortex cells do not divide, but enlarge making the stem swell.
The dividing cells that will become adventitious roots are densely cytoplasmic and have prominent nucleoli.
The cell divisions are polar and the cells line up in parallel rows.
The second stage of root formation is the organization of dividing cells into a root initial. At about ten days after sticking the cutting, a root meristem is starting to organize in the root initial.
This can be seen by the concentration of cells staining red forming a typical root tip shape. At this time, there is no evident vascular connection between the cutting and the new developing root.
By day 15, the developing organs have reached the root primordia stage. At this stage, the root primordium has organized a well defined root meristem and a root cap can usually be seen.
The vascular system of the root primordium is forming and reaching for a connection with the xylem and phloem in the parent stem.
Notice how the root primordia are breaking through the sclerenchyma ring and pushing aside the swollen cortex cells.
Eventually, adventitious roots will push their way through the cortex and epidermis and emerge to be functional roots. For most softwood and herbaceous cuttings, the root emergence stage is complete between 20 and 30 days.
Obviously, this is the stage where visible roots can be seen on the cutting. It also has a secure vascular connection with the cutting's stem.
Although, the cutting is not ready to transplant yet, usually the cutting does not need as frequent misting at this stage.