Biology Today — December 2017

Activity of Vascular Cambium

Rays initials

• Ray initials form the radial or horizontal system of secondary tissues.


• Ray initials of cambium ring divide by tangential divisions and add new cells at places
  forming narrow bands of parenchyma which passes through both secondary xylem
  and secondary phloem. These are called medullary rays. They are one to few layers
  in thickness and several layers in height. Medullary rays are responsible for radial
  conduction of solutes and maintain connection between pith and cortex.

Fusiform initials

• Fusiform initials form axial system of secondary vascular tissues.


• These initials divide on outerside to form new cells which remain meristematic for
  a definite period and later get differentiated into secondary phloem. The primary
  phloem gets crushed.


• They divide on inner side to form secondary xylem.
  Fig.: L.S. of vascular cambium

Secondary Vascular Tissues

• The secondary phloem comprises of sieve tubes, companion cells, phloem parenchyma and band patches of bast fibres.
  Many textile fibres of commercial value as jute, hemp, flax, etc., are the bast fibres of secondary phloem.


• The secondary phloem does not grow in thickness as the primary and older secondary phloem present on outer side gets
  crushed with the development of new functional phloem.


• The secondary xylem consists of scalariform and pitted vessels, tracheids, numerous wood fibres arranged usually in radial
  rows and some wood parenchyma.


• Since the cambium is more active on inner side, the xylem increases more rapidly in bulk than the phloem and forms a compact
  mass. Due to continued formation of secondary xylem and pressure exerted by it, the cambium, phloem and surrounding
  tissues are gradually pushed outwards. Though some primary tissues get crushed but the primary xylem remains intact in
  or around the centre.


• The bulk of secondary xylem is referred to as wood and its width increases with the age of the plant.

Annual Rings

• The annual or yearly growth of wood in the form of distinct
  concentric rings are called annual rings. Distinct annual rings are
  formed in areas with distinct climatic variations i.e., experiencing
  two seasons, which affects the uniform activity of cambium
  throughout the year.


• The age of the plant can be determined by counting the number
  of growth rings, formed in it.


• As each annual ring corresponds to one year’s growth.

Formation of annual rings

• In regions with distinct seasonal variations, i.e., favourable (spring
  or rainy) and unfavourable (autumn, winter or dry summer) the
  cambium activity is not uniform. During favourable seasons when
  the temperature, sunshine and humidity is optimum, the need for
  sap transport is acute to increase in production and activity of
  foliage leaves. Therefore, cambium is more active forming a greater
  number of vessels with wider cavities or larger, pitted vessels.

Fig. : Drawing of a 4-year old woody stem

showing the growth increment as annual rings.

Ray system

Year 1 2 3

Axial system

Vascular cambium

Primary growth Secondary growth

Bark