Study of Primary Plant Structures (With Diagram)

In this article we will study about primary structures of plant with the help of suitable diagrams.

(A) Stem of Cucurbita sp. (Fig. 6.1):

The stem is wavy in outline and thus shows distinct ridges and furrows.

The stem shows the following plan of arrangement of tissues in transverse section:

1. Epidermis:

It is the outermost layer of the cross-section. It is a single-layered zone made of compactly arranged tabular or barrel-shaped cells. The cells are cuticularied and some of them possess multicellular un-branched hairs. Stomata may be present in the young stem.

2. Cortex:

This is multi-layered and differentiated into three zones: hypodermis — which is composed by collenchymatous cells forming conspicuous patches, particularly at the ridges; parenchymatous cortex layer, and innermost layer of starch sheath layer or endodermis.

3. Stele:

It is centrally located, bounded on the outer side by the endodermal layer of cortex. It contains vascular bundles, sclerenchymatous band, parenchymatous cells in the inter-fascicular regions, and hollow pith. The sclerenchymatous bands along with the underlying parenchyma layers form the pericycle.

The vascular bundles occur in two rings, outer ring usually consists of smaller vascular bundles located opposite the ridges. These are leaf-trace bundles. Inner ring of bundles occurs against the furrows. Thus the stele is a dictyostele.

4. Vascular bundles:

The vascular bundles are typically conjoint, open, bicollateral, having two patches of phloem and two strips of cambium on either side of xylem. Xylem tissues are arranged in the end-arch manner.

5. Pith:

It is disorganized in the early stage, thus forming a hollow cavity.

Comments on Anatomical Features:

The specimen shows the features of a typical di-cotolyledonous stem.

The reasons are:

(a) Presence of single-layered, parenchymatous epidermis with frequent multicellular hairs;

(b) Presence of collenchymatous hypodermis, parenchymatous cortex and starch sheath layer just outside the stele;

(d) Presence of endarch xylem.

There is a special feature of vascular bundles, each bundle is bicollateral open type, i.e., xylem tissue is bounded on both sides with cambium and phloem tissues.

(B) Stem of Helianthus sp. (Fig. 6.2):

In T.S. view of sunflower stem (Helianthus annuus, family Compositae), following tissue arrangement is noticed from periphery to centre:

1. Epidermis:

It is the outermost uniseriate, parenchymatous cell layer. It is distinctly cuticularised and also possesses multicellular hairs.

2. Cortex:

It is situated just below the epidermis and extends up to stele. The outer layers of cortex is collenchymatous, called hypodermis. The parenchymatous cortex is located below hypodermis. A few glands are also present in the cortex. These are resin ducts. The innermost cortical layer is called starch sheath or endodermal layer.

3. Stele:

All the tissues occurring internal to the starch sheath constitute the stele. It is a dissected siphonostele, consisting of vascular bundles arranged in the form of a ring.

4. Vascular Bundles:

They are typically conjoint collateral and open type — xylem being internal and phloem external. A strip of lateral meristem, cambium, is present in between. Xylem is endarch. Each vascular bundle is provided with sclerenchymatous bundle cap.

5. Pith:

At the centre of the stem axis, conspicuous parenchymatous pith is well-marked.

Comments on Anatomical Features:

The anatomical features of sunflower stem thus reveal the characters of a typical dicotyledonous stem.

The reasons are:

(a) There is presence of uniseriate epidermis with distinct cuticle and multicellular hairs.

(b) There is conspicuous hypodermis and endodermis in the cortical layer.

(d) The vascular bundles are conjoint, collateral and open.

(e) There is presence of a conspicuous pith.

(C) Stem of Leonurus sp. (Fig. 6.3):

The stem is square in cross-section.

It shows the following plan of arrangement of tissues:

1. Epidermis:

It is single-layered, composed of tabular parenchymatous cells joined end to end. The outer walls are strongly cuticularised. Many multicellular un-branched hairs develop on the epidermis. Stomata are present.

2. Cortex:

It is differentiated into three zones:

(a) Next to epidermis occurs hypodermis composed of collenchyma cells. These cells aggregate densely at the four corners of the stem,

(b) There are a few layers of thin-walled parenchyma cells just internal to collenchymatous hypodermis. These cells contain abundant chloroplasts and appear as a green belt under microscope,

(c) The innermost layer of cortex is the starch sheath, composed of barrel-shaped compactly-arranged cells with abundant starch grains.

3. Stele:

It is externally bounded by starch sheath layer. It is made of vascular bundles, intrastelar parenchymatous tissues and pith. A few layers of sclerenchyma forming a continuous band occur next to the starch sheath. This is pericycle or perivascular tissue.

4. Vascular Bundles:

The vascular bundles are conjoint collateral and open. Xylem is endarch i.e. protoxylem occurs towards centre and metaxylem towards circumference.

5. Pith:

It is parenchymatous and large.

Comments on Anatomical Features:

The specimen shows the features of a typical dicotyledonous stem.

The reasons are:

(a) There is a single-layered parenchymatous epidermis with multicellular hairs.

(b) Collenchymatous hypodermis, parenchymatous cortex, and starch sheath layer are present.

(d) There is presence of a large parenchymatous pith.

The hypodermal collenchymatous tissues aggregate densely at the four corners of the stem so as to serve as diagonally placed I-girders for withstanding flexion.

(D) Stem of Zea sp. (Fig. 6.4):

The stem is circular in cross-section.

Tissue arrangement is:

1. Epidermis:

It is single-layered, composed of small tightly set cells. The outer walls are cuticularised and hairs are absent.

2. Cortex:

There is no direct zonation in the cortical region. Vascular bundles are scattered all over the parenchymatous ground tissue. There is a thin band of hypodermal sclerenchyma layer just beneath the epidermis.

3. Stele:

It is an atactostele. The bundles occurring towards the periphery are smaller in size and more crowded, whereas those at the central region are larger in size and more spaced.

4. Vascular Bundles:

The vascular bundles are conjoint and closed. Xylem occurs in the form of the letter ‘Y’ — the two metaxylem vessels with wider cavities occur along the two arms, and protoxylem vessels — usually one or two — with narrow cavities are at the base.

In mature bundles, xylem elements undergo more and more lignification and the lowest protoxylem elements disintegrate forming a lacuna or cavity known as protoxylem cavity. Each bundle is surrounded by sclerenchyma cells as a bundle sheath.

5. Pith:

It is not distinguishable from the ground tissue.

Comments on Anatomical Features:

The specimen shows the features of a typical monocotyledonous stem.

The reasons are:

(a) There is a cuticularised epidermis which is devoid of hairs.

(b) There is no demarcation of cortex and stellar zone. Vascular bundles are scattered all over the ground tissue.

(c) Each vascular bundle is conjoint, collateral and closed. Xylem is arranged in the form of the letter Y. There is protoxylem cavity in mature bundles.

(d) Each bundle is covered by a distinct bundle sheath.

(e) Pith is not differentiated.

(E) Stem of Aquatic Grass (Fig. 6.5):

A transverse section through the stem of an aquatic grass shows the following plan of tissue arrangement:

1. Epidermis:

It is uniseriate, composed of a row of compactly arranged cells. The cells have cuticularised outer wall.

2. Ground Tissues:

It is differentiated into outer (two to three layers) sclerenchymatous hypodermis and parenchymatous ground tissue which contains scattered vascular bundles.

3. Vascular Bundles:

These are fairly large in number and remain scattered in the ground tissue. Peripheral bundles are smaller than central bundles. The bundles are collateral and closed. Xylem is arranged in the form of the letter ‘V’. Protoxylem cavity is present. Phloem is present in-between the two arches of xylem. Each bundle remains surrounded by a sclerenchymatous sheath.

4. Pith:

It is not distinguishable. The central part is hollow in T.S. of mature stem.

Comments on Anatomical Features:

The specimen shows the features of a typical monocotyledonous stem.

The reasons are:

(a) There is a uniseriate, cuticularised epidermis without hairs.

(b) The cortex is not distinguishable. Vascular bundles are scattered in the ground tissue.

(d) Pith is not distinguishable.

(F) Inflorescence Scape of Canna sp. (Fig. 6.6):

The floral axis of Canna shows features almost identical to a monocotyledonous stem, though a bit different from those of maize stem.

The tissues are:

1. Epidermis:

It is the uniseriate protective zone with cuticularised outer walls.

2. Ground Tissues:

A few layers of parenchyma occur next to epidermis forming a narrow cortex. It is immediately followed by a band of chloroplast-containing cells. This band is referred to as chlorophyllous tissue. Patches of sclerenchyma are present in the chlorophyllous tissue here and there. The remaining portion is parenchymatous containing scattered vascular bundles.

3. Vascular Bundles:

The bundles are collateral and closed. Each of them has a small amount of xylem and phloem. Sclerenchyma cells remain as patches on both sides of the bundles as caps.

4. Pith:

It is not distinguishable.

Comments on Anatomical Features:

The specimen shows the features of a monocotyledonous stem.

The reasons are:

(a) Presence of uniseriate cuticularised epidermis.

(b) Presence of massive ground tissues with scattered vascular bundles.

(d) Presence of chlorophyllous tissues in the outer cortical region.

(G) Root of Cicer Sp. (Fig. 6.7):

It shows the following plan of arrangement of tissues from the epidermis to the centre of the organ:

1. Epidermis or Epiblema:

It is typically uniseriate outermost zone consisting of tabular living cells. There are some unicellular root hairs arising from epidermal layer as prolongations of the cells. Cuticle and stomata are completely absent.

2. Cortex:

It is relatively simple and homogeneous, forming a massive zone which consists of unspecialized parenchyma cells with conspicuous intercellular spaces. The innermost layer of cortex is endodermis. It is of universal occurrence in roots and consists of compactly arranged barrel-shaped cells forming a distinct zone surrounding the stele. The endodermal cells possess casparian thickenings on radial walls.

3. Stele:

It includes the vascular tissues and intrastelar ground tissues. Next to endodermis there lies a layer of thin-walled parenchyma cells forming the pericycle.

4. Vascular Bundles:

The vascular bundles are radial. Xylem and phloem occur in separate patches arranged on alternate radii, intervened by small patches of parenchyma cells. The bundle is tetrarch and xylem is arranged in exarch pattern, i.e., protoxylem arranged towards the periphery.

5. Pith:

It is not properly distinguishable, thus the stele is a protostele.

Comments on Anatomical Features:

The specimen shows the features of a typical dicotyledonous root.

The reasons are:

(a) There is a non-cuticularised single layered epiblema, with unicellular hairs arising as prolongations of epidermal cells.

(b) There is a massive parenchymatous cortex with distinct innermost endodermal layer.

(d) Bundle is tetrarch and xylem is arranged in exarch manner.

(e) Pith is almost absent.

(H) Root of Arum (Fig. 6.8):

The root of arum in transverse section shows the following pattern of tissue arrangement:

1. Epidermis or Epiblema:

It is uniseriate, composed of a row of tabular cells attached end to end without having intercellular spaces. There is frequent occurrence of root-hairs from epiblema.

2. Cortex:

It is quite massive and mainly consists of un-specialised parenchyma with profuse intercellular spaces. The innermost layer of cortex is the endodermis. It is composed of thickened cells (Casparian strips) and some thin-walled cells (passage cells).

3. Stele:

It is the central cylinder and consists of radially arranged vascular strands and intrastelar ground tissues. Uniseriate pericycle, made of thin-walled parenchyma cells, occurs next to endodermis.

4. Vascular Bundles:

Xylem and phloem remain arranged alternately on separate radii, xylem being typically exarch. The vascular bundle is polyarch.

5. Pith:

The central part of the stele is occupied by a fairly large parenchymatous pith with copious intercellular spaces.

Comments on Anatomical Features:

The specimen shows the features of a typical monocotyledonous root.

The reasons are:

(a) There is a distinct epidermal layer with unicellular root-hairs which are prolongations of the cells.

(b) The cortex is massive and parenchymatous. The innermost endodermal layer is composed of cells with capsarian strips and passage cells.

(c) The stele is composed of radially arranged xylem and phloem strands surrounded by a distinct parenchymatous pericycle layer.

(d) The vascular bundle is polyarch and exarch with reference to xylem arrangement.

(e) There is a distinct parenchymatous pith.

(I) Aerial Root of Orchid (Fig. 6.9):

The aerial roots of epiphytic orchid (Vanda sp.) show a characteristic anatomical organization.

Tissue arrangement is:

1. Velamen:

It is the outermost spongy layer. It consists of a few layers of compactly set dead cells, which often have a silvery outer coating. Velamen layer is actually a multiseriate epidermis, especially adapted to serve as an absorbing tissue. The outermost layer of velamen is known as limiting layer. The cells are empty with porose and variously thickened walls.

2. Cortex:

The outermost layer of cortex composed of a row of thick-walled cells — exodermis, contains some thin-walled cells called passage cell. The main cortex consists of thin walled parenchyma cells having intercellular spaces among them. The innermost layer of the cortex is endodermis that consists of compact barrel shaped cell. The endodermis completely encircle the stele.

3. Stele:

Pericycle is uniseriate, made of thick-walled cells; only the cells just lying internal to passage cells of the endodermis are thin-walled. It has radial stele. A good number of xylem and phloem groups occur alternately in the stele along different radii. Conjunctive tissues surrounding the phloem groups are sclerenchymatous.

4. Pith:

The central portion is occupied normally by the parenchymatous pith, but these cells may undergo sclerosis.

Comments on Anatomical Features:

The specimen shows the features of a monocotyledonous root.

The reasons are:

(a) Presence of thick-walled exodermis as an alternative to epidermis.

(b) Presence of parenchymatous cortex.

(d) Presence of radial arrangement of xylem and phloem tissues. The vascular bundle is poly arch.

(e) Presence of distinct parenchymatous pith.

In addition to these, there is a distinct layer of velamen tissue, which acts as a moisture-absorbing tissue needed as an epiphytic adaptation.

(J) Leaf of Mango (Fig. 6.10):

The leaf of mango is dorsiventral and has reticulate veination.

A thin section through the lamina shows the following pattern of tissue arrangement:

1. Epidermis:

There are two epidermal layers — upper (on abaxial surface) and lower (on adaxial surface) epidermis. Each layer is uniseriate, being composed of a row of compactly-set tabular cells and cuticularised externally. Stomata occur mostly on the lower epidermis.

2. Mesophyll (Ground Tissues):

The ground tissue forming mesophyll is differentiated into palisade and spongy layers. The palisade layer occurs towards upper epidermis and is composed of columnar cells. The spongy layer occurs towards the lower epidermis and is composed of loosely arranged roundish cells. Mesophyll cells contain abundant chloroplasts.

3. Vascular Bundles:

The vascular bundles are collateral and closed. They are located in the mesophyll. Each bundle is composed of xylem and phloem, the former occurring towards upper epidermis and the latter towards the lower side. Each bundle is bounded by a bundle sheath, which is parenchymatous in nature.

Comments on Anatomical Features:

The specimen shows the features of a typical dicotyledonous dorsiventral leaf.

The reasons are:

(a) There are two epidermal layers of which lower epidermis contains stomata.

(b) There is a distinct differentiation of mesophyll cell layers into spongy mesophyll layer and palisade mesophyll layers.

(K) Leaf of Banyan (Fig. 6.11):

The leaf of banyan is more or less similar to other dicotyledonous dorsiventral leaves.

The tissue arrangement is:

1. Epidermis:

The upper epidermis is multiseriate, being made up of a few layer of cells. Cystoliths are present in this layer. The lower epidermis is uniseriate. Both upper and lower epidermal layers possess distinct cuticle externally. Stomata are present in lower epidermis and have sub-stomatal air chambers.

2. Mesophylls:

It is differentiated into palisade and spongy cells. Palisade cells are located in the upper region while spongy cells towards lower epidermis. Mesophyll cells contain chloroplasts.

3. Vascular Bundles:

The bundles are collateral and closed, with xylem lying on the upper and pholem on the lower sides. They remain surrounded by parenchymatous bundle sheaths with or without extension. Larger bundles have patches of sclerenchyma at the two ends.

Comments on Anatomical Features:

The specimen shows the feature of a typical dorsiventral leaf.

The reasons are:

(a) There are two epidermal layers — upper multiseriate and lower uniseriate with stomata.

(b) The mesophyll cells are of two major types — palisade and spongy cells.

The cuticularised multiseriate upper epidermis with cystoliths is the characteristic of many species of the genus Ficus to which banyan belongs.

(L) Leaf of Bamboo (Fig. 6.12):

This section through the leaf of bamboo reveals the features of a monocotyledonous isobilateral leaf.

The tissue arrangement is:

1. Epidermis:

There are two epidermal layers: upper epidermis possesses a number of conspicuous bulliform cells. And the lower one bears stomata. Both the layers are covered with cuticle. Sharply pointed and stiff hairs are present.

2. Mesophyll:

It is composed of compactly arranged cells without differentiation into spongy and palisade zones. The cells are of palisade type, some large air spaces are present in the middle.

3. Vascular Bundles:

These are collateral and closed ones which remain arranged in parallel series. Xylem occurs on the adaxial and phloem on the abaxial sides. Bundles are surrounded by parenchymatous bundle sheath layers. Sometimes associated with sclerenchyma cells on both sides as bundle sheath extensions.

Comments on Anatomical Features:

The specimen shows the features of an isobilateral leaf.

The reasons are:

(a) There are two epidermal layers. Both are cuticularised and single-layered. The upper one possesses bulliform cells (a Graminaceous feature) and lower one has stomata.

(b) The mesophyll cells are compactly arranged, without showing any differentiation into palisade and spongy cells.

(M) Leaf of Date Palm (Fig. 6.13):

A section through a leaf of date palm (Phoenix sylvestris) reveals the following structure:

1. Epidermis:

There are two distinct epidermal layers — upper and lower epidermis. Both are cuticularised and also have stomata. A layer of parenchyma cells with scanty chlorophyll occurs just internal to both the epidermal layers. These are distinctly different from the mesophyll.

2. Mesophyll:

It is composed of more or less isodiametric cells with small intercellular spaces. There is no differentiation into palisade and spongy cells. Patches of sclerenchyma occur more or less in parallel series towards both the upper and lower epidermis, as I-girders for withstanding shearing stress.

3. Vascular Bundles:

The bundles are collateral and closed. Each bundle has xylem on the upper and phloem on the lower side. The bundles are of two types: large and small ones. The large bundles have patches of high thick-walled sclerenchyma on the two edges, whereas the small bundles remain surrounded by sheaths of parenchyma cells which have no chloroplasts.

Comments on Anatomical Features:

The specimen shows the features of a typical isobilateral leaf.

The reasons are:

(a) There are two distinct epidermal layers, upper and lower. Both the epidermal layers have cuticle and stomata.

(b) Mesophyll cells are not differentiated in palisade and spongy types.

(d) Presence of I-girders in the mesophyll cell layers.