Biology Today — December 2017

Stelar Growth by Vascular Cambium

Against the protoxylem, the cambium forms distinct and widening

radial bands of parenchyma that constitute the primary medullary rays.

These rays extend upto secondary phloem. These are made up of two

parts- xylem or wood ray (present in xylem) and phloem ray (present in

secondary phloem). They help in radial conduction of substances.

The wavy band of cambium soon becomes circular and thus, a

cambium ring is produced. The entire cambial ring becomes actively

meristematic and gives rise to secondary xylem on the inner side

and secondary phloem on the outer side.

The conjunctive tissue on the

inner side of phloem becomes

meristematic and gives rise to

cambial strip. The number of

cambial strips is equivalent to

number of phloem bundles.

The cells of cambium strips (vascular cambium) divide repeatedly to produce

new cells both towards inner as well as outer side. The cells produced

towards inner side (centripetally) differentiate into secondary xylem

elements and those produced towards outer side (centrifugally) differentiate

into secondary phloem. Subsequently, the cells of pericycle lying towards

outer side of protoxylem divide by tangential division. It divides and forms

a strip of cambium there as well and joins with the cambium strips formed

earlier on either side of xylem. Thus, a continuous wavy band of cambium is

formed, extending over the xylem and down the phloem. Secondary growth

commences with the activity of this cambium band.

Epiblema

Cortex

Primary phloem

Conjunctive

parenchyma

Metaxylem

(primary xylem)

Protoxylem

Endodermis

Pericycle

Primary dicot root

Endodermis

Epiblema

Primary

phloem

Primary xylem

Cambium

Pericycle

Cortex

Secondary phloem

(Consists of sieve

tubes, companion

cells and phloem

parenchyma)

Secondary xylem

(Consists of vessels,

tracheids and xylem

parenchyma)

Endodermis

Epiblema

Rays

Primary phloem

(Gets crushed)

Secondary phloem

Cambium

Pericycle

Cortex

Cork

Rays

Phellogen

Secondary

phloem

Primary

xylem

Secondary

xylem

Primary phloem Cambium

(Gets crushed)

Endodermis

Epiblema

Primary

phloem

Cambium

Pericycle

Cortex

Extrastelar Growth by Cork Cambium

• Phellogen consists of only one type of initial cells.


• Cells of phellogen divide both towards the outside as well as inside forming cork or phellem and secondary cortex or
  phelloderm, respectively.


• It results in the formation of outer protective covering consisting of multilayered cork, cork cambium and multilayered
  secondary cortex, which collectively constitute periderm.


• The secondary cortex of root does not contain chloroplasts and is very much compressed being thin walled. It ultimately
  gets disorganised and sloughs off. Endodermis also meets the same fate while epiblema dies out.


• Lenticels may also form in places.


• The bark of root forms a thin protective covering and is not extensive.