Biology Today — December 2017

Anomalous growth in dicot stem

In a stem, numerous vascular bundles occur in a ring embedded in conjunctive tissue scattered in the centrally located

pith. In the beginning, there are numerous scattered primary vascular bundles. These bundles are collateral and open.

The cambium of the bundles is active and individual bundles show little amount of secondary growth. This activity

stops after some time. These bundles come to lie in the pith and are now called as medullary bundles. Secondary

growth begins later with the development of a new cambium outside the stele. This cambium cuts off conjoint and

collateral vascular bundles on the outer side. These are secondary bundles which remain embedded in the large

amount of conjunctive tissue formed by the cambium. Many rings of vascular bundles are formed which remain embedded

in the conjunctive tissue and their phloem consequently gives an appearance of included phloem or phloem islands at number

of places.

Cortex Cuticle

Collenchyma

(hypodermis)

Stomata Epidermis

Parenchyma Starch sheath

Secondary phloem

Phloem

Pith

Secondary xylem

Xylem

Conjunctive tissues

Cambium

Cambium

Primarymedullarybundle

Fig.: Amaranthus : Detailed structure of a part of T.S. of stem

Epidermis
Hypodermis
Parenchyma
Starch sheath
Secondary phloem

Secondary xylem

Primary medullary

bundle

Pith

Conjunctive tissue

Cambium

Fig.: T.S. of stem of Amaranthus (diagrammatic)

Anomalous Secondary Growth in Monocots

Though secondary growth is rare in monocots, it is commonly seen in woody monocotyledons like Dracaena, Yucca,

Aloe, Agave, etc. In such arborescent monocot stems, the secondary cambium grows in hypodermal region, and

forms conjunctive tissue and patches of meristematic cells. The latter grows into secondary vascular bundles. In Dracaena

at a very late stage during the development, a wide zone of secondary meristem (cambium) develops outside the

vascular bundles in the parenchymatous regions. This meristematic tissue cuts off vascular bundles on its inner side.

These are concentric (amphivasal) in contrast to the primary bundles which are collateral. The amount of

parenchymatous ground tissue also increases, therefore, the diameter of the stem. The cambium (meristematic zone)

originates near the leaf primordia. The life of this zone or layer is limited. It stops functioning after sometime and

the adjacent cells take over. Another important feature in contrast to cambium of dicotyledons is that cambium in

Dracaena cuts off both xylem and phloem on its inner side while on its outer face very little amount of parenchyma

is produced.