Comprehensive Essay on the Primary and Secondary Growth in Plants

The saprophyte of a vascular plant begins its life history as a fertilized egg that divides and becomes a multicellular, elongated embryo. Such an embryo has a shoot apex at one tip and a root apex at the other. At these tips certain cells called apical meristems remain permanently embryonic.

They continued to divide, and new cells formed by them are added behind each tip to the embryonic tissues already present. The whole embryo continues to elongate in this manner. The embryonic tissues behind the apical meristems soon begin to specialize as adult stem and root tissues, and as their distance from the tips increases and tissues become progressively more mature.

Thus, a lengthwise view of a shoot or a root exhibits an orderly sequence of zones corresponding to the stages each adult stem or root tissue has passed through during its development. The concentrically arranged adult tissues are, from the outside inward, epidermis, cortex, endodermis (in roots), and stele. The stele consists of epicycle (in roots), phloem, xylem, and pith.

In the root, the root-hair cells of the epidermis are usually in a distinct zone some distance behind the root tip. Roots hairs are temporary structures; ahead of the root- hair zone they have not yet developed and behind it they have already disappeared.

The root-hair therefore advances as the root tip advances. Also present in a root cap, externally. Formed by the apical root meristem, a root cap is an important adaptive device. As the root tip advances, hard soil grains would soon macerate unprotected meristem tissue. But in the presence of a root cap the cap cells wear off instead and the growing tip is shielded effectively. New cap cells continue to be formed by the root meristem.

The roots of most tracheophytes are without pith, and the steles in such cases are called protosteles. The simplest type of protostele is a naplostele, in which xylem, phloem, and epicycle have a cylindrical arrangement. Most tracheophyte roots have actinosteles, or variants of protosteles in which the xylem has a star like appearance in cross section.

In the stem, the epidermis is cutinized and contains paired guard cells with stomata. The endodermis and epicycle are reduced or absent, but pith is present in most cases. Steles with pith occur in a large number of variant forms. In the class of gymnosperms and one subclass of angiosperms (the dicots), so-called dietyosteles are encountered.

Here the xylem and phloem tissues are arranged as distinct and separate vascular bundles, and these are grouped in more or less circular patterns in the stem. Such a circle of bundles surrounds the pith, which is continuous with the cortex outside the ring of bundles.

In a second subclass of angio sperms (the monocots), the steles are atactosteles. Here xylem and phloem again form vascular bundles, but they are scattered randomly throughout the stem. It is therefore often difficult to distinguish precisely between pith and cortex.

Primary growth: leaves and branches

Stem and root growth proceed indefinitely as a result of the continuous production of new cells at the shoot and root tips. By contrast, leaf growth is usually limited in time, for a leaf generally does not possess an apical meristem of its own. A leaf bud forms embryonic tissue just below the shoot apex. Sometimes a single cell but more often several cells give rise to the leaf bud.

These embryonic cells divide repeatedly, most divisions occurring along the margins of the expanding and flattening blade. Concurrently a column of future xylem and phloem tissue, a so-called leaf trace, branches away from the stele of the stem and grows into the leaf.

Two leaf types can be distinguished according to the amount of vascular tissue formed by a leaf trace. In a microphyll, the amount of xylem and phloem is equivalent to at most a single vascular bundle. Such leaves occur only in the primitive subphyla of traceophytes. By contrast, the subphylum of pteropsids is characterized by megaphylls, in which the xylem and phloem are equivalent to numerous vascular bundles.

Where a leaf trace of a megaphyll branches away from the stele of the stem, a small region of the stele just above this branch point does not specialize as vascular tissue. Such regions are leaf gaps, which do not form during the development of microphylls.

When their tissues are mature, the leaves of the vast majority⁷ of tracheophytes have attained their final sizes and do not grow thereafter.

A microphyll consists of a leaf blade only. A mature megaphyll usually includes a petiole, in a thin basal stalk that attaches the leaf to the stem: two stipules, small appendages that grow out near the base of the petiole in many species; and a lamina, the leaf blade itself. The epidermis of a leaf is continuous with that of the stem.

As in the stem the leaf epidermis is cutinized and contains green guard cells with stomata. The interior of the leaf consists of parenchymatous mesophyll, normally the chief food-producing tissue of the plant; all mesophyll cells contain chlorophyll.

In most tracheophytes the mesophyll is organized as two distinct zones. Just underneath the upper epidermis in horizontally placed leaves and underneath the whole epidermis in most upright and needle-shaped leaves, mesophyll cells form compact layers, or palisade parenchyma.

Elsewhere mesophyll consists of spongy parenchyma, loose layers honeycombed extensively with air spaces. These connect with one another and lead to the exterior through open passages in the palisade tissue and the stomata. Such a structural arrangement permits the greater part of every mesophyll cell to come into direct contact with fresh external air.

Embedded in mesophyll are leaf veins, composed of supporting fibre tissue and of xylem and phloem bundles that are continuous with the vascular tissues of the stem. Veins give mechanical support and carry nutrients to and from all parts of the leaf.

The veins usually form networks (as in decoct angiosperms) or parallel strands (as in monocot angiosperms). In external form, leaves are most often blade­like, needle-like, or scale-like. Leaves with single blades are said to be simple, those with more than one blade, compound.

Leaves are arranged along a stem in different patterns. In an alternative pattern, single leaves are attached at successive stem levels and the leaf bases mark out a spiral winding up along a stem. The geometric characteristics of such spirals are usually quite distinct for given species.

Leaves are opposite if two of them grow out at the same stem level, and they are whorled if more than two arising at the same level. The regions of the stem where leaves are attached are called nodes; the leaf-free regions between nodes are internodes.

In deciduous plants leaves that have dropped off in the fall leave permanent leaf scars on the stem. Such plants also develop bud scales, which cover the apical shoot meristem during the winter. These scales are modified leaves or leaf parts, and they form a terminal bud, on a dormant stem in winter condition. When fall off and leave densely placed bud-scale scars on the elongating stem. By counting the number of stem regions where such scars occur it is often possible to determine the age of a plant.

Stems usually produce many lateral branches. These arise from branch buds, developed in the apical shoot meristem in the leaf axel. This is the upper angle where a leaf joins the stem. Wherever a leaf is formed, a branch bud forms in the leaf axel as well. Branch buds leave their own branch gaps in the stele of the parent stem.

Branch buds often do not mature immediately. Some remain dormant for many years, and others may not develop at all. In wintering stems dormant branch buds are usually clearly visible just above leaf scars.

When a branch bud does mature, it develops an apical shoot tip of its own and grows in every respect like the parent stem. Evidently, an important difference between a branch and a leaf is that one does and the other does not acquire a growing tip in the bud stage.

Roots are without nodes, but branches form nevertheless. However, the process of branch development differs from that in the stem. At varying distances behind the apex of the main root, cells in localized regions of the pericycle divide and form a pad of tissue, a so-called root primordial.

During its later development such a primordium pushes through the peripheral tissues of the primary root. By the time it emerges through the epidermis, a root meristem and a root cap have been formed. A stele with vascular tissue then matures, and this tissue become continues with corresponding ones of the main root. At this stage the lateral root is fully established and continues to grow like a main root.

Secondary Growth

The whole organization of the tracheophyte body described up to this point represents the product of primary growth; all body parts are direct derivatives of the apical meristems and the original immature tissues of the embryo.

Primary growth essentially permits extension in length, and any increase in the thickness of stems and roots results only means of increasing body size. However, numerous tracheophytes are capable of growing not only in length but also in thickness, through lateral increase of cell number.

These plants undergo processes of secondary growth, superimposed on a continuing primary growth. Apart from comparatively enormous increases in stem and root girth, the large-scale result of secondary growths is the development of bark and of wood. Wood tends to be formed in relatively large quantities, and new layers are added each year to those already accumulated.

Plants of this type develop as trees and become recognizably woody in appearance. Primary growth actually gives rise to “wood”, or primary xylem, too, but in the vast majority of cases this xylem is formed in such small amounts that the plant is left in herbaceous condition; and if a distinctly woody plant is to develop, abundant wood must be formed by secondary growth. Thus the phrase “woody plants” refers largely to plants in which secondary growth occurs (specifically angiosperms)

In these woody plants young shoots and roots begin to develop, as in all other cases, through primary growth. The plant also continues to elongate through primary growth at each tip, and the regions immediately behind the tips maintain the characteristic primary structure of nonfood roots and stems.

More specifically, the primary’ root pattern is actinostelic and the primary stem pattern is largely dietyostelic. But in order regions, well behind each tip, these primary patterns later become transformed to secondary⁷ ones. The early shoots and later growing tips thus remain “green” even in woody plants. Since leaves bud off near the shoot apex and do not have growing tips of their own, they do not participate in secondary development at all.

The transformation of roots and stems from primary to secondary states is brought about by secondary meristems, or cambia. Two kinds of cambium develop a vascular cambium and a cork cambium. Each arises from different primary tissues, and the process of formation differs somewhat in root and stem.

In a root, the vascular cambium forms between the primary xylem and phloem in the stele. A layer of cells here remains permanent embryonic and relatively unspecialized. This layer becomes the vascular root cambium, which ultimately surrounds the primary root xylem completely.

In a stem part of the vascular cambium again forms from a cell layer between primary xylem and phloem. Since these tissues in the dictostelic stem from a ring of vascular bundles, the cambium layer between the xylem and them is interrupted between neighbouring bundles. Such interruptions soon disappear, however, for a layer of parenchyma cells between the bundles acquires the properties of a cambium.

The vascular stem cambium thereby becomes a complete tube, continuous with the corresponding tube in the root. In the course of the further elongation of the stem-root axis through primary growth at the apical tips, the open-ended cambial tube lengthens as progressively more cambium develops behind the stem, the cells of the vascular cambium continue to divide and bud off new cells toward both the inside and the outside.

In this manner whole layers of cells continue to be deposited at both sides of the cambium. Most of the cells that are budded off toward the inside of the cambial layer soon mature as the various components of xylem tissue; and cells that are budded off toward the outside become components of phloem tissue. Vascular tissues so generated by cambium represent secondary xylem (or wood proper) and secondary phloem.

These secondary tissues are traversed in places by radial xylem rays and phloem rays. Such rays are strands of parenchymatous tissue that are generated by small patches of cells (ray intials) in the cambium. New cells budded off by these patches both toward the inside and the outside do not become vascular xylem and phloem, but instead they grow out as columns of cells that form the xylem and phloem rays, respectively. The rays function in lateral transport of nutrients in stems and roots.

As secondary xylem continues to be formed in successive continues to be formed in successive concentric layers inside the cambial tube, it cannot grow too far inward for this space is already occupied by the primary Style and the pith.

An outward expansion will therefore take place, and the thickness of the stem or the root will increase. Similarly, as secondary phloem develops outward it increases the thickness of stem or root still more. Indeed, as secondary phloem presses increasingly against primary phloem, cortex, and epidermis, these primary tissues must ultimately rupture; for being adult and thus no longer able to grow, they cannot keep pace with the ever-expanding girth of the stem or the root.

The smallest amounts of secondary tissue are always near the apical tips, where cambial activity is just beginning; the largest amounts have accumulated at the stem-root juncture, the region that has grown for the longest period and is the oldest. This is therefore the region of greatest girth, and from here the stem tapers up and the root tapers down.

The second of the cambial tissues is the cork cambium. This single layer of cells develops from the epicycle in roots and from the cortex or the phloem in stems. Like the vascular cambium, the cork cambium produces new cell layers toward the inside and outside. Layers budded off toward the inside develop as cork parenchyma (or phelloderm).

Layers generated toward the outside form cork (or phellem). During their maturation cork cells deposit heavy sobering coats on their walls, and the cell interiors then disintegrate; mature cork is wholly nonliving. (The cork cambium and its products, phelloderm and phloem, are collectively also known as periderm).

Cork cells are usually packed close together an arrangement that makes the outer covering of woody plant quite impervious to water and air. At various places, however, the cork cambium produces loosely arranged cork cells separated by intercellular spaces. Such spongy regions are lenticels. They permit gas exchange between the atmosphere and the interior living tissues of root or stem.

Cork first develops where the epidermis and the cortex have ruptured as a result of the outward expansion of xylem and phloem. Later, after epidermis and cortex have torn away completely, a continuous layer of cork comes to surround the entire outside surface of stem and root. Further increases in stem and root diameter then because a rupturing and flaking off of the original cork, but new cork is produced in its place. This new tissue in turn ruptures and flakes off, and the cycle of new formation and flaking off repeats indefinitely. In roots the epicycle itself soon ruptures and flakes off, and secondary phloem then becomes the chief source for the regeneration of cork cambium, as in the stem.

All tissues outside the vascular cambium are collectively called bark. In a cut section of a tree the main tissues then are, concentrically from the outside inward: cork; the microscopically thin layer of cork cambium; cork parenchyma; secondary phloem; the microscopic layer of vascular cambium; and wood, which fills the space within the ring of vascular cambium. In a stem section a microscopic accumulation of pith is in the very centre.

Older phloem, right below the surface of a tree trunk, continually flakes off as the trunk thickens. At any given time, therefore, only a thin rind of young phloem is present in bark. Similarly, only young xylem is functional.

Older xylem near the centre of a trunk in time gradually blocks up with resins and gums, and water conduction through these channels is then no longer possible. Such central regions are called heartwood. The core of a tree can therefore be hollowed out without interfering with xylem conduction. But the outer, young wood of a tree, called the sapwood, must remain intact if a tree is to remain alive.

In an older tree growing in the temperate zone, the xylem vessel laid down during spring generally have a larger diameter than those formed in summer. In spring, melting snow provides the tree with much water, and wider conducting channels formed at that season accommodate the greater flow.

This alternation of narrow summer xylem and wider spring xylem is recognizable with the naked eye as a concentric series of dark and light bands, or annual rings. The number of rings indicates the age of a tree. Moreover, from the comparative width of spring and summer rings it is often possible to estimate the amount of rainfall, hence general climatic conditions, during the past season as far back in time as the tree has lived.

Through the secondary growth processes described, a young, green shoot is slowly transformed to a tall, thick, tapering woody tree. As noted, the bulk of a tree trunk is nonliving, and a woody plant expends large amounts of energy and materials every year in producing new tissues that soon become nonliving too. In this respect an herbaceous plant is far more economical; for the tissues formed by primary growth just suffice to maintain life and they serve the plant as long as it lives.

To be sure, even a tiny herb is built on a lavish structural scale when compared with a microscopic unicellular alga floating in the ocean. But such lavishness is the price of survival on land; of all photosynthetic organisms really only the tracheophytes have become completely successful as terrestrial types. In this success the evolution of vascular tissue has been one basic factor. Another has been the evolution of seeds, a topic examined later in another context.

Psilopsids

Subphylum Psilopsida: leaves are microphylls; roots absent, absorption through rhizoids; psilotum, tmesipteris; 3 species.

These most primitive of all living vascular plants are xerophytes. Two species of Psilotum grow in tropical and subtropical regions of the Americas, and Tmesipteris is found in Australia and on some Pacific islands. Some of the plants live as epiphytes on trees, others in soil.

The traits of psilopside are remarkably similar to those of their fossil ancestors. Stems are partly horizontal rhizomes, underground in the no epiphytes, partly upright and aerial. Unicellular rhizoids grow from the horizontal portions of a stem.

The aerial portions have a heavily cutinized epidermis with sunken stomata, and they bear tiny microphylls. These are without stomata or vascular tissue in Psilotum but do contain a single strand of vascular tissue as well as stomata in Tmesipteris.

Lycopsids

Subphylum Lycopsida: leaves are microphylls; roots present; Lycopodium, club mosses ground pines; selaginella, spike mosses, resurrection plants; lsoetes, quillwort’s; 900 species.

Many ancestral glycosides were large woody trees, but their present-day relatives are invariably small and no woody. These plants still range in respectable numbers from the tropics to north temperate regions, and the ground pine Lycopodium in particular is relatively common. Many glycosides are creepers with horizontal rhizomes, others are erect, and some are ‘epiphytes’.

The roots of Lycopodium are frequently adventitious, the pericylce of the stem being the tissue from which such roots arise. In erect forms, adventitious roots sometimes originate near the shoot apex and grow down right through the stem cortex. The microphylls contain air spaces in the mesophyll and stomata on both surfaces.

Selaginella lives largely in damp, shady places in the tropics. Stems are often partly horizontal partly upright, and the epidermis is cutinized and without stomata. Young plants develop typical roots, but nearly all later roots form from so-called rhizophores.

These are stem like down growths from forks in aerial stems. The portions of a rhizophore that enter the ground reorganize structurally and functionally as a root.

The microphylls have stomata, and each leaf also bears aligule, a small tuft of tissue attached to the upper leaf surface near the juncture with the stem. Neither the function nor the evolutionary history of ligules is known. Quillworts are the only surviving lycopsids that by virtue of a special cortical cambium are still capable of secondary growth. They do not become woody, however.

The plants live as deciduous perennials in marshy areas, and their underground stems are corns. From these projects dense clusters of erect microphylls, which give quillwort’s part of their English name as well as a superficial resemblance to patches of lawn grass.

Sphenopsids

Subphylum Sphenopsida: leaves are microphylls: stems with nodes bearing whorls of leaves; roots present; equisetum (horsetails, scouring rushes); 25 species.

All surviving sphenopsids are members of the single genus Equisetum, found from the tropics to the temperate zone. The plants have underground rhizomes and aerial – stem parts that have nodes. Narrow microphyll grow out in whorls at such nodes.

The stems are usually ribbed lengthwise strengthened with silica deposits, and hollow internally. Stomata occur between the ridges of the stem and in the leaves. Roots are largely adventitious and grow out along the rhizome.