Ch 10Secondary Growth — Class 11 Botany, concept-first.

A plant organ's growth begins at its apical meristems - the growing tips of root and shoot - which drive an increase in length, called primary or longitudinal growth.

Secondary growth in a dicot stem is carried out jointly by the vascular cambium and the cork cambium. The vascular cambium first forms as a continuous ring inside the stem and begins cutting off new c…

The vascular cambium is the lateral meristem responsible for producing all of a dicot stem's secondary vascular tissue - secondary xylem (wood) laid down toward the inside of the ring and secondary ph…

The vascular cambium of a dicot stem does not arise from one single source - it is assembled from two structurally different starting tissues that eventually join into one continuous ring.

Once the vascular cambial ring is complete, its cells resolve into two functionally distinct kinds of initials, distinguishable by their shape and orientation.

When the vascular cambial ring is active, its initials divide periclinally (parallel to the stem's surface) and cut off new daughter cells on both faces at once - toward the inside and toward the outs…

Secondary xylem, more commonly known simply as wood, is the tissue produced by the vascular cambium on its inner face, and its study by preparing microscope sections is called xylotomy.

Wood is also classified by age and function into sap wood and heart wood. Sap wood (also called alburnum) is the pale-coloured, physiologically active outer zone of the secondary xylem - the youngest,…

The vascular cambium's activity is not constant through the year - it is controlled by physiological and environmental factors, and in temperate regions where the climate itself varies sharply through…

The vascular cambial ring produces secondary phloem, also called bast, on its outer face - the counterpart, on the food-conducting side, to the secondary xylem produced on the inner face.

Whenever a stem or root thickens through secondary growth, its original epidermis (and, often, the primary cortex beneath it too) is no longer able to keep stretching to cover the widening surface, an…

Phellem, commonly called cork, is the outermost component of the periderm - a protective tissue made of dead, non-living cells with walls impregnated with suberin (a waxy, waterproofing substance), cu…

Phellogen, more commonly called the cork cambium, is the secondary lateral meristem responsible for producing the entire periderm.

Phelloderm, also called secondary cortex, is the innermost component of the periderm - a living tissue resembling ordinary cortical parenchyma, produced centripetally (inward) by the phellogen as its…

Phellem and phelloderm are both produced by the same meristem - the phellogen - yet end up structurally and functionally opposite: one dead and protective, the other living and nutritive.

Two specialised terms describe periderm-related tissues beyond the basic phellem-phellogen-phelloderm unit.

Although the vascular cambium and cork cambium are both secondary lateral meristems that appear during a stem or root's secondary growth, they differ in almost every respect - their origin, the kind o…

'Bark' is the everyday term applied to everything outside the vascular cambium of a stem - the periderm, the cortex, the primary phloem and the secondary phloem together, considered as one functional…

A lenticel is a raised opening or pore that develops on the epidermis or, later, on the bark of a stem or root during secondary growth.

Roots undergo secondary growth for essentially the same reason stems do - the aerial parts of the plant need a stronger, thicker anchoring and conducting base as the plant grows - and the overall proc…

Beyond the wavy-versus-circular difference in how the cambial ring first appears, dicot stem and dicot root secondary growth differ in several other, related ways, summarised in the table below.

The various secondary tissues covered across this chapter - secondary xylem, secondary phloem, the rays that run through them, and the periderm's phellem and phelloderm - do not all trace back to one…

Not every dicot or monocot that undergoes secondary growth follows the single, continuous vascular-cambium-plus-cork-cambium pattern described in the sections above.

Timber is wood derived from felled tree logs, and it is put to enormously wide use, especially in carpentry and house construction.

Seasoning of wood is the process by which the moisture content naturally present in freshly cut timber is removed, and it is an essential step before timber is used, since drying increases the wood's…

Air seasoning is the process of removing moisture from cut timber without resorting to any artificial source of heat.

Kiln seasoning is the artificial counterpart to air seasoning - the removal of moisture from cut timber by artificial means, carried out inside an enclosed chamber rather than in the open air.

Once timber has been seasoned, its visual and structural quality is further described using three related but distinct terms.

The chapter's Evaluation section closes with twenty questions covering the full span of secondary growth: multiple-choice questions on cambial seasonal activity, why monocots generally do not thicken,…

Sample papers and previous-year board questions for this subject.