Anatomy of Flowering Plants

From outside a plant looks like one solid object, but inside it is a set of tissues. A tissue is a group of cells that have a common origin and usually do the same work together. The first big division is between tissues whose cells are still dividing and tissues whose cells have stopped dividing and settled into a fixed job. The dividing ones are called meristematic tissues, or meristems. The settled ones are called permanent tissues.

A meristem cell has a very recognisable look. It is small, thin walled, roughly square or many sided in outline, packed tightly against its neighbours with no gaps between cells, and it has dense cytoplasm, a large prominent nucleus, and either no vacuole at all or a very small one. Learn this description carefully, because a mature parenchyma cell is almost its opposite: bigger, with one huge vacuole and a thin layer of cytoplasm pushed against the wall. If you can see large vacuoles in a section, you are not looking at a meristem.

Apical meristems sit at the tip of the root and at the tip of the shoot. They build the primary plant body, so they are the reason a plant gets taller and its roots go deeper. There is a useful detail here. As a leaf is formed and the stem elongates, some of the cells that the shoot apical meristem leaves behind stay meristematic, and this leftover group is the axillary bud. So an axillary bud is really a saved packet of apical meristem sitting in the angle between the leaf and the stem, and it is able to grow later into a branch or a flower.

Intercalary meristem lies between mature tissues, typically at the base of the internodes or at the base of the leaf. It is common in grasses. This is why a lawn or a grazed field grows back from the bottom after the tops have been cut or eaten: the growing zone was never removed. Apical and intercalary meristems are grouped together as primary meristems, because they appear early in the life of the plant and their product is the primary plant body.

Lateral meristems appear later, and they are cylindrical, so on a cut surface they show up as a ring rather than a point. The two lateral meristems are the vascular cambium and the cork cambium. They add cells sideways, towards the inside and towards the outside, so they increase the girth of a stem or root instead of its length. Because they appear later in life, they are called secondary meristems, and everything they build is called secondary tissue. Keep the pairing clean in your mind: apical and intercalary give length, lateral gives thickness.

Once a meristem cell stops dividing and takes up a fixed size, shape, wall thickness and function, we say it has become differentiated. Sometimes a living differentiated cell can go back to dividing, which is called dedifferentiation. That is exactly what happens when parenchyma cells inside a stem turn back into interfascicular cambium or into cork cambium. When such a dedifferentiated tissue loses its dividing power again and settles down, the process is called redifferentiation.

A permanent tissue is called simple when every cell in it is of the same type, and complex when the tissue is a team of different cell types working together. There are exactly three simple permanent tissues, and the fastest way to tell them apart is to ask one question: where is the cell wall thickened, and what is it thickened with?

Parenchyma is the commonest and the least specialised tissue, and it usually forms the bulk of any soft plant organ. Parenchyma cells are typically isodiametric, which means roughly the same width in every direction; in shape they may be spherical, oval, round, many sided or elongated. Their walls are thin and made of cellulose. The cells may be packed closely together or may have small spaces between them. Parenchyma is alive at maturity. It stores food and water, takes part in secretion, and helps in healing wounds. When parenchyma cells contain chloroplasts and carry out photosynthesis, the tissue is called chlorenchyma. Cortex, pith, medullary rays and the pulp of a fleshy fruit are all parenchyma.

Collenchyma is the flexible support tissue. It occurs in layers just below the epidermis in dicotyledonous plants, either as a continuous ring or in separate patches. Its cells may be oval, spherical or many sided, and their defining feature is that the wall is extra thick at the corners, where the deposit is made of cellulose, hemicellulose and pectin. Note that the thickening material is not lignin. There are no intercellular spaces in collenchyma. The cells are living and may contain chloroplasts, so collenchyma can also make food. Its main job is mechanical support in parts that are still growing, such as a young stem or the petiole of a leaf. Because the walls are not lignified, the tissue can stretch and bend, which is exactly what a growing organ needs.

Sclerenchyma is the hard, rigid support tissue. Its cells are long and narrow with thick, lignified secondary walls that often carry a few pits, and they are usually dead at maturity with no living protoplast left inside. Sclerenchyma comes in two forms. Fibres are thick walled, elongated and pointed at the ends, and they generally occur in groups in different parts of the plant. Sclereids are spherical, oval or cylindrical, extremely thickened and dead, with a very narrow cavity or lumen left in the middle. Sclereids are what your tongue feels as grittiness in the pulp of guava, pear and sapota; they are also found in the fruit walls of nuts, in the seed coats of legumes and in the leaves of tea. Sclerenchyma gives mechanical strength to organs that have finished growing, which is why you find it in mature stems and in hard seed coats rather than in soft growing tips.

Put simply: parenchyma is thin walled and does the general living work, collenchyma is corner thickened and gives bendable support to growing parts, and sclerenchyma is uniformly lignified, dead and gives rigid support to mature parts.

Xylem conducts water and dissolved minerals and also gives strength to the plant body. It has four components: tracheids, vessels, xylem fibres and xylem parenchyma.

Tracheids are elongated, tube like cells with thick lignified walls and tapering ends. They are dead and have no protoplasm at maturity. Vessels are long cylindrical tubes made of many cells placed end to end; each such cell is a vessel member, and the common walls between successive members carry perforations, so the whole file behaves as one continuous pipe. Vessel members also have lignified walls, a large central cavity and no protoplasm. The presence of vessels is a characteristic feature of angiosperms. Xylem fibres have very thick walls and their central lumen is almost closed up; they may be septate or aseptate and their job is support. Xylem parenchyma is the only living component; the cells are thin walled and store food such as starch and fat, along with substances like tannins, and they also help move water sideways through the radial rows called medullary rays.

The first formed xylem is called protoxylem and it has narrower elements; the later formed xylem is metaxylem and it has wider elements. Where the protoxylem sits relative to the metaxylem has a name of its own. In a young stem the protoxylem lies towards the centre, that is towards the pith, and the metaxylem towards the periphery, and this arrangement is called endarch. In a root it is the other way round: protoxylem lies towards the periphery and metaxylem towards the centre, which is called exarch. A quick memory hook is that in roots the water is entering from outside, so the oldest and narrowest xylem faces outwards.

Phloem transports food, mainly sugars made in the leaves. In angiosperms it has four components: sieve tube elements, companion cells, phloem parenchyma and phloem fibres.

Sieve tube elements are long tube like cells arranged end to end in a vertical file. Their end walls are perforated and are called sieve plates. A mature sieve tube element is unusual: it is alive, but it has lost its nucleus, and it keeps only a thin peripheral layer of cytoplasm around a large vacuole. Because it has no nucleus of its own, its activity is controlled by the nucleus of the neighbouring companion cell. Companion cells are specialised parenchyma cells that lie beside the sieve tube element and stay connected to it through pit fields in their common longitudinal walls. Phloem parenchyma cells are elongated and tapering with dense cytoplasm and a nucleus, and they store food and substances such as resins, latex and mucilage; note that phloem parenchyma is absent in most monocotyledons. Phloem fibres, also called bast fibres, are sclerenchymatous, generally absent in primary phloem and present in secondary phloem, and they lose their protoplasm at maturity. The commercial fibres of jute, flax and hemp are phloem fibres. As with xylem, the first formed phloem is protophloem with narrow sieve tubes and the later formed phloem is metaphloem with wider sieve tubes.

One comparison is worth holding on to: gymnosperms do not have sieve tube elements and companion cells at all. Their phloem contains sieve cells and albuminous cells instead.

Instead of listing tissues one by one, it is easier to group them by position. On this basis the plant body has three tissue systems: the epidermal tissue system, the ground or fundamental tissue system, and the vascular tissue system.

The epidermal tissue system is the outermost covering of the whole primary plant body, and it includes the epidermal cells, the stomata and the epidermal appendages. The epidermis is usually one cell thick. Its cells are parenchymatous, with a small amount of cytoplasm lining the wall and a large vacuole inside. On the outside the epidermis of aerial parts carries a waxy layer called the cuticle, which cuts down water loss; the cuticle is absent from roots.

Stomata are the openings in the epidermis that regulate transpiration and the exchange of gases. Each stoma is bounded by two guard cells. In dicots the guard cells are bean shaped, while in grasses they are dumb-bell shaped, and this shape difference alone can help you name a section. The outer wall of a guard cell is thin and the inner wall, the one facing the pore, is thick; because of this uneven thickening the guard cell bulges outwards when it takes in water, and the pore opens. Guard cells contain chloroplasts, unlike ordinary epidermal cells. In some plants a few epidermal cells next to the guard cells become different in shape and size, and these are called subsidiary cells. The pore, the two guard cells and the surrounding subsidiary cells together are called the stomatal apparatus.

The epidermis also produces appendages. On roots, some epidermal cells stretch out into root hairs, which are unicellular and greatly increase the surface for absorbing water and minerals. On stems, the epidermal hairs are called trichomes. Trichomes are usually multicellular, may be branched or unbranched, soft or stiff, and are sometimes secretory. They help reduce water loss from the surface. Learn the pair carefully: a root hair is one cell, a stem trichome is usually many cells.

The ground tissue system is everything except the epidermis and the vascular bundles. It is made of the simple tissues you already know. In a primary stem or root it appears as the cortex, the pericycle, the pith and the medullary rays, and it is mostly parenchyma with collenchyma or sclerenchyma wherever support is needed. In a leaf, the ground tissue is the thin walled, chloroplast filled tissue called mesophyll.

The vascular tissue system is made of the complex tissues, xylem and phloem, grouped into vascular bundles. Bundles are classified in two ways. If xylem and phloem lie on different radii, alternating with each other around a circle, the bundle is radial, and this is what roots have. If xylem and phloem lie on the same radius, one behind the other, the bundle is conjoint, and this is what stems and leaves have; in a conjoint bundle the phloem is usually only on the outer side of the xylem. Conjoint bundles are further called open when a strip of cambium lies between the xylem and the phloem, as in dicot stems, and closed when there is no cambium, as in monocot stems. This one word, open or closed, decides whether the stem can ever show secondary growth.

Dicot root. Going inwards from the surface you meet the epidermis, which in a root bears root hairs and is also called the epiblema; then the cortex, several layers of thin walled parenchyma with intercellular spaces; then the endodermis, the innermost layer of the cortex, made of a single ring of barrel shaped cells with no intercellular spaces. On the tangential as well as the radial walls of the endodermal cells there is a deposit of the water impermeable waxy substance suberin in the form of Casparian strips. Just inside the endodermis is the pericycle, a few layers of thick walled parenchymatous cells; lateral roots start from here, and during secondary growth part of the vascular cambium also arises from here. The vascular bundles are radial, with usually two to four patches of xylem, and between the xylem and phloem lies parenchyma called conjunctive tissue. The pith is small or hardly visible. Everything inside the endodermis, that is the pericycle, the vascular bundles and the pith, together forms the stele.

Monocot root. The layers are the same in name, so the differences are what matter. A monocot root has more than six xylem bundles, described as polyarch, and it has a large, well developed pith. Monocot roots do not undergo secondary growth at all.

Dicot stem. The outermost layer is the epidermis with a thin cuticle, sometimes bearing trichomes and a few stomata. The cortex has three zones: an outer collenchymatous hypodermis that gives mechanical strength, then rounded thin walled parenchyma with clear intercellular spaces, and then the innermost cortical layer, the endodermis, whose cells are rich in starch grains and which is therefore called the starch sheath. Inside that, on the outer side of each phloem group, the pericycle appears as half moon shaped patches of sclerenchyma. The vascular bundles are conjoint, open and endarch, and they are arranged in a ring. Between neighbouring bundles lie the medullary rays, a few radial rows of parenchyma. The centre is filled with parenchymatous pith.

Monocot stem. The hypodermis here is sclerenchymatous, not collenchymatous. The ground tissue is a large continuous mass of parenchyma with no separation into cortex, pericycle and pith. Numerous vascular bundles are scattered through this ground tissue instead of forming a ring; each bundle is surrounded by a sclerenchymatous bundle sheath, and each is conjoint and closed. The bundles near the periphery are generally smaller than those towards the centre. Phloem parenchyma is absent, and a water containing cavity is usually present in the bundle on the protoxylem side.

Dorsiventral (dicot) leaf. Both surfaces have an epidermis covered by a cuticle, and the lower (abaxial) epidermis generally has more stomata than the upper (adaxial) one, which may have none. Between the two epidermal layers is the mesophyll, and in a dicot leaf it is clearly divided into two: an upper palisade parenchyma of elongated cells standing vertically side by side, and below it a spongy parenchyma of oval or rounded cells that are loosely arranged with many intercellular spaces and air cavities. Vascular bundles run in the veins and the midrib, their size matching the size of the vein, and each is enclosed in a layer of thick walled bundle sheath cells.

Isobilateral (monocot) leaf. The general plan is similar, but stomata are present on both surfaces in roughly equal numbers, and the mesophyll is not divided into palisade and spongy layers. In grasses, some of the upper epidermal cells along the veins are enlarged, empty and colourless; these are the bulliform cells. When they are full of water and turgid the leaf blade stays spread out, and when they lose water they become flaccid and the leaf rolls inwards, which reduces water loss. Because monocot leaves have parallel venation, the vascular bundles are of nearly the same size all across the blade, except in the main veins.

Primary growth makes a plant longer. Secondary growth makes it thicker, and it is carried out by the two lateral meristems: the vascular cambium and the cork cambium. It occurs in the stems and roots of most dicotyledons and of gymnosperms.

Forming the cambial ring. In a young dicot stem, cambium is present only in patches, as a single layer lying between the xylem and the phloem of each vascular bundle; this is the fascicular or intrafascicular cambium. Later the cells of the medullary rays that lie next to these patches become meristematic again and form the interfascicular cambium. The two kinds join up, and the result is a continuous ring of cambium going all the way round the stem.

What the cambial ring does. Once active, the ring cuts off new cells on both of its faces. The cells cut off towards the inside, that is towards the pith, mature into secondary xylem; the cells cut off towards the outside mature into secondary phloem. The cambium is generally more active on the inner side than on the outer, so far more secondary xylem is made than secondary phloem. Over the years this secondary xylem builds up into a compact mass that forms the greater part of the stem, and it is what we call wood. The primary and secondary phloem, being pushed outwards against the surface, get gradually crushed, while the primary xylem stays more or less intact near the centre. At certain points the cambium also produces narrow bands of parenchyma that run radially through both the secondary xylem and the secondary phloem; these are the secondary medullary rays, and they carry food and water sideways.

Annual rings. The cambium does not work at the same speed all year. In spring it is very active and produces many xylem elements with wide vessels; this wood is light in colour and low in density and is called spring wood or early wood. In winter the cambium is less active and produces fewer elements with narrow vessels; this wood is darker and denser and is called autumn wood or late wood. Because the two alternate, a cut stem shows alternating concentric bands, and one band of spring wood together with the following band of autumn wood makes one annual ring. Counting the annual rings in a cut trunk gives an estimate of the age of the tree, and since ring width depends on the growing conditions of that year, the rings also record something about past climate.

Heartwood and sapwood. In an old tree the inner, older part of the secondary xylem turns dark brown because organic substances such as tannins, resins, oils, gums and aromatic compounds are deposited in it. This dark central region is the heartwood, also called duramen. Heartwood no longer conducts water; its role is to give mechanical strength, and the deposits also make it resistant to attack by insects and microbes, which is why it is prized as timber. The outer, lighter coloured region of the secondary xylem is the sapwood or alburnum, and it is the part that is still functional in conducting water and minerals upwards.

Cork cambium and bark. As the stem keeps thickening, the outer cortex and the epidermis are stretched and torn, so a new protective layer is needed. A second lateral meristem, the cork cambium or phellogen, therefore develops, usually in the cortex. It is only a couple of layers thick and is made of narrow, thin walled, nearly rectangular cells. Like the vascular cambium, it cuts off cells on both sides: the cells produced on the outside become cork or phellem, and those produced on the inside become the secondary cortex or phelloderm. Cork cells become impervious to water because suberin is deposited in their walls, and they are dead at maturity. Phellogen, phellem and phelloderm taken together are called the periderm.

Bark is a non technical but useful term for all the tissues lying outside the vascular cambium, so it includes the secondary phloem as well as the periderm. Bark formed early in the season is called soft bark and that formed towards the end of the season is called hard bark. At certain places the phellogen makes loose parenchymatous cells outwards instead of cork; these push through and rupture the surface, leaving lens shaped openings called lenticels, through which gases are exchanged between the inner tissues and the air. Lenticels occur in most woody trees.

In roots the story is slightly different. The vascular cambium of a dicot root is entirely secondary in origin: it arises from the parenchyma lying just inside the phloem groups, together with the parts of the pericycle that lie outside the protoxylem. Since the phloem groups and the protoxylem points alternate around the root, this new cambium is at first a wavy ring, and it becomes circular later, once it starts producing secondary tissue.