Last week was a brief overview of suckers, or shoots that form from adventitious buds on roots. Today I want to pick up from the Sucker Shoot Development section in that post, with more detail and some diagrams to illustrate this development more clearly.

Last week I wrote that the adventitious bud which becomes a sucker shoot arises from the pericycle (picture here). I did that so as not to introduce too many new botanical words at once — it can be overwhelming and headache-inducing, I know!

While technically correct, more accurately, the pericycle actually develops into tissue called the phellogen, also known as cork cambium, and it’s this that the bud arises from. But first let’s back-track a bit before going further.

The previous posts covering root anatomy longitudinally and cross-sectionally focused on the primary growth of a root. Primary growth, in botany, is the growth at a root’s (or stem’s) tip, where cell division and elongation occur.

Secondary growth, in botany, is the growth that causes roots (and stems) to thicken. During this growth, the pericycle forms two types of lateral (side) meristem tissue, the outer cork cambium and the inner vascular cambium. (Fig. 1 below shows this more clearly.) Both of these continue to thicken the root for as long as they keep producing new cells.

The vascular cambium is responsible for the bulk of this thickening. It produces the secondary phloem (food) tissue on its outer side and the secondary xylem (water) transport tissue on the inner side. (What we call ‘wood’ is xylem by the way!)

The cork cambium produces a protective layer called the periderm in botany, but commonly referred to as bark. (’Bark’ is actually the periderm plus the secondary phloem — see Fig. 1 below. Bark includes all the tissue outside the vascular cambium, in other words.) The cork cambium replaces the root epidermis (picture here), which is ruptured by this secondary growth.

The periderm is actually three layers (Fig. 1 below): the outside cork (also called phellum, of mostly mature, dead cells); the middle cork cambium (or phellogen); and the inner phelloderm.

[And yes, the material commonly called ‘cork’ is indeed the cork (phellum) layer of a particular tree, the cork oak Quercus suber.]

Still with me?! Let’s now expand on the Sucker Shoot Development section from last week — this will be so much easier to follow along now you know the above.

Please bear in mind that the following images show only the top half of a longitudinal section of root.

As mentioned earlier, a sucker shoot arises from an adventitious bud in the cork cambium — a bud not originating from stem or root apical (apex) meristem (undifferentiated) tissue.

This bud is nothing more than a small bundle of meristem tissue itself at the beginning, and with no vascular (transport) connection to the root at all. It gradually specialises to form an apical meristem of its own at one end, and vascular tissue at the other.

Primordial leaf tissue then develops from the apical meristem and the vascular tissue develops at the other end to connect with the root’s vascular tissue. (Fig. 2.) Procambium tissue, which will become the primary phloem and xylem of the shoot, also develops and joins with the root cambium.

The bud can now access the mother tree’s food and water resources for future growth and development.

Fig. 3 shows the progress some time later, whereby the bud is larger and more leaf tissue has formed. The vascular cambium has laid down two new rings of wood which enclose the bud’s vascular tissue. Two rings in this case implies two years of growth, but this is purely for illustrative purposes and could be a shorter or longer period.

As the bud’s procambium tissue is joined with the root’s vascular cambium, this enclosure of wood results in the procambium forming a complete cylinder within the bud. This begins to differentiate into protophloem (first-forming phloem) and protoxylem (first-forming xylem).

Fig. 4 shows the final stage of development, with more wood laid down and the bud now an above-ground shoot.

Over time lateral roots will develop on the mother root this sucker is attached to, and it is these roots, and not the mother root, that would sustain a sucker if removed. The detached piece of mother root must contain sufficient number and size of these secondary roots if the sucker is to survive and grow independently as a new tree.