‘A Resume – an explanation of a poorly investigated condition’

 

Despite the recognition of the existence of this structure, as a significant part of stem development, for at least a century, there has been no significant investigation in terms of its influence in stem cutting propagation until recently and then only fairly cursorily in limited anatomical and physiological studies. Its correlation with the development of adventitious roots and their emergence has received scant attention and requires some relevant investigation (if only from an economic standpoint) as it is evident that in some woody plants it is a significant factor in limiting the potential rooting of stem cuttings.

 

During the secondary thickening phase of the vascular tissues in the development of the stems of woody plants, there is a concurrent development of a sheath of cells within the inner limit of the cortex – in developmental terms it represents the outer perimeter of the stele. It is surmised that this is a protective mechanism against pathogens and, when thickened, also provides some structural rigidity. It is variously described as the Pericycle, Pericyclic Sheath or Sclerenchyma Sheath.

                                                

In many plants this sheath develops no further than consisting of parenchyma cells but in many woody plants, structurally, it consists of a layer cells which are principally of sclerenchyma cells (as fibres or sclereids); this may be several cells thick but can vary in the density of its structure and texture – ie it can vary in the degree to which parenchyma gaps occur and the degree to which the sclerenchyma cells are thickened.

 

It surrounds and encloses the vascular bundle complex. Its apparent significance in propagation is in the way that it may act as a barrier to root emergence (or may influence and hinder root initiation) as it is normally within the vascular complex that adventitious root initiation occurs and the root primordia develop.

 

In many (usually regarded as easy to root) species the sheath is not sufficiently ‘strong’ to impede root penetration to the exterior and the root (with its root cap) will rupture the sheath and push through readily. However there is evidence that a ‘strong’ sheath can impede root emergence and roots will deviate downwards when they encounter the barrier, and emerge through the base of the cutting.

 

Variation in ‘density’ and ‘mesh’ - ie the continuity of the sheath and hence the occurrence of parenchyma (or sclereid free) gaps, thus, more often than not equates to, and reflects in, ‘ease of rooting’ – and a complete and dense sheath usually heralds poor rooting.

 

Although there has been little (or virtually no) detailed observation on the effects of the speed of growth of a new stem on the proportion of parenchyma gaps, it is reasonable to propose that the improved ease of root initiation associated with this activity, has caused a decrease in the density of the sclerenchyma cells and an increase in the occurence of the sclereid free gaps.

 

Recently a similar effect has been reported for the effects of Etiolation treatments to stems of Carpinus betulus fastigiata – the indications are that, after such treatment, the sheath contains a greater proportion of sclereid free gaps than are found in non-etiolated shoots and that this can be correlated with an improved rooting response – the response was evident for three months (1996, Maynard and Bassuk, J. Amer. Soc. Hort. Sci. 121(5) 853-860).

 

Adventitious roots may not be able to penetrate to the exterior therefore they may have to turn down and emerge from the base of the cutting – this is often evident in Hardy hybrid Rhododendrons – when using a single thick extension shoot as a stem cutting; conversely a thin shoot emanating from an umbrella of shoots – usually developed as a result of disbudding and which, as a result, ‘grow faster’ will develop roots laterally through the stem. The thick shoot as a cutting also tended to respond to a shallow, basal slice wound or scoring – a process which reduces the integrity of the sheath – and enhanced rooting when propagation was attempted.

 

The following paper has reported that there is a direct correlation between the rooting response and the proportion of sclereid free gaps in the sheath for two species of North American Oaks – Quercus bicolor had more sclereid free gaps than Q. macrocarpa and also demonstrated a much greater root production on stem cuttings (2008, Amissah, Paolillo and Bassuk, J. Amer. Soc. Hort. Sci. 133(4) 479-486).

 

A similar response has also been reported in a comparison of two apple rootstock varieties in which M5 roots readily and M25 does not and that this is directly related to the proportion of sclereid free gaps in the sheath.