The initiation and development of adventitious roots on a stem cutting is a fundamental process to achieving a successful conclusion to this mode of vegetative plant propagation and although scientists have been intrigued by this process for two or three centuries – the practicalities still appear to be a matter of debate. The questions which arise can simply be catalogued as:-

Where do they arise?

What causes the initiation?

How do they develop?

When are they initiated?

 

The scenario presented here represents a resume of the current position and conclusions which are garnered from the limited basket of current observations.

 

Although some woody plants (eg Poplar, Willow, Apple, Jasmine and Flowering Currant) are capable of initiating and developing root initials in stems as a normal growth process, it is exceptional and, in general, adventitious rooting is not initiated in stems unless there is an external stimulus to do so – this stimulus is provided by the separation of the stem from the parent and it would appear that the wound thus caused is the primary agent in stimulating this activity.

 

When a stem is separated from the parent plant (as in making a cutting) it creates a wound at the cut base of the stem. The natural process is for this wound to be healed. This happens with the development, initially, of a necrotic plate of cells which suberise, this is followed, internally, by the production of a layer of parenchyma cells which also thicken and form a barrier, described as callus, of thickened cells. This activity then produces a further response, which is presumably ‘auxin’ controlled, to initiate root production. This initiation occurs in parenchyma cells in the region of the vascular bundles (within or surrounding) and inside the pericyclic sheath.

 

These cells begin the process of de-differentiation which implies that they no longer function in their current role and revert to a meristematic condition. This is followed by a differentiation as the cells reconfigure their function and future development into root initials. This can be observed microscopically as these cells, which normally divide lengthwise clinally (parallel with the circumference), begin to divide at right angles (anticlinally) and a three dimensional group of cells becomes apparent – the root initial.

 

The sites at which this process actually happens are diverse. Although these sites are usually inside the pericyclic sheath and close to the vascular tissue – predominantly in the phloem ray cells - but can be found in the inner cortex, the cambium, medullary rays and almost anywhere that suitable parenchyma cells are found. In any particular stem cutting the initials are not necessarily confined to any one type of site but may be found with different sites of origin.

 

Once the root initial is created it then proceeds to differentiate into a root primordium. Eventually this primordium differentiates as a root, together with its root cap, and penetrates the pericyclic sheath, through the cortex and bark to the exterior. Concurrently the root is also connecting to the vascular tissue so that the root has a normal and developed connectivity with the conducting tissues of the stem.

 

The final issue concerns how long this process will take and there is no definitive answer - as there appears to be a wide divergence in the speed at which the initiation is triggered and how fast the development will occur.

 

Although it is, logically, reasonable to suppose that the rooting response is initiated by the activity of plant growth regulating substances, it is certainly not clear how this happens or what is the cue for a particular site to become active. Presumably the process is controlled by particular genes – but this takes the process to another area of comprehension!