In this article, we propose to discuss about the secondary growth in monocotyledonous stem.
Normally the vascular bundles of the monocotyledonous stems are closed ones. Thus due to absence of the cambium” they lack secondary growth in thickness and the vascular system is wholly composed of primary tissues.
The bundles remain irregularly scattered in the ground tissues, forming an atactostele, where the limits of cortex and other ground tissues can be hardly discerned.
Some monocotyledons belonging to the family Liliaceae, mainly the arborescent ones like Dracaena (Fig. 643), Yucca, Cordyline, Agave, Aloe and others exhibit a peculiar type of secondary increase in thickness, an account of which is given here.
In the regions which have ceased to elongate some cells occurring outside the vascular bundles become meristematic and form the cambium. The secondary meristem originates in the cortex, in fact, deep layers of cortex or pericycle, though it is difficult to locate those zones in many stems.
The cambial cells are fusiform or rectangular in shape. Instead of forming phloem and xylem on the outer and inner sides, as in normal condition, the cambial cells go on dividing and producing secondary tissues on the inner side first, and later small amount of new tissues are cut off on the outer side as well.
Those formed on the inner side differentiate into oval-shaped vascular bundles and radially arranged parenchyma cells. These parenchyma cells in which the vascular bundles remain embedded are said to constitute the conjunctive tissue.
The radial arrangement of the parenchyma cells of conjunctive tissue is due to their origin by tangential divisions of the cambial cells. So they may be easily distinguished from the irregularly arranged parenchyma of the primary ground tissues. They may be thin-walled or thick- walled.
A few derivatives of the cambium divide longitudinally, initially anticlinally, then periclinally and even haphazardly to form xylem and phloem elements of the secondary vascular bundles. These differ from the primary ones in presence of small amount of
phloem and in absence of annular and spiral protoxylem elements.
The secondary bundles are mostly amphivasal, some of them may be collateral as well. The small amount of phloem consists of short sieve tubes, companion cells and parenchyma. The xylem is made of only tracheids, usually with scalariform thickening and small amount of xylem parenchyma which have lignified walls.
The tissues cut off by the cambial cells on the outer side are scanty in amount and are parenchymatous in nature. The primary bundles are comparatively larger. They are also mostly amphivasal or rather rarely collateral ones.
In Aloe, cocoanut and royal palm periderm is formed in the same manner as in the dicotyledons. A special type of protective tissue is produced in the majority of the monocotyledons with secondary increase in thickness.
Parenchyma cells in the deeper layers of cortex divide periclinally several times and produce radially arranged layers of cells, suggesting something like a storied meristem. The series of cells thus formed undergo suberisation and differentiate into cork cells. This tissue is called storied cork, mainly’ because of its storied appearance.
Considerable thickening which is often noticed at the bases of many palms (Fig. 644) is not due to the activity of a cambial layer, but it is really a case of protracted or long-continuing primary growth.
The apical meristem or shoot apex produces only a part of the primary body —-a central column of parenchyma and vascular strands. Another meristem occurs just beneath the leaf primordia forming a mantle-like tissue region. This is known as primary thickening meristem.
The cells of the same divide periclinally and form rows of cells which differentiate into ground parenchyma traversed by procambial strands. The latter, ultimately develop into vascular bundles. There are further enlargement and divisions of the ground parenchyma, causing increase in thickness.
Thus the primary thickening meristem produces the main bulk of stem tissues, viz., ground parenchyma and most of the vascular bundles. The thickening is due to the activities of the two meristems stated above.
