Anatomy of Flowering Plants – the Angiosperms

Anatomy of Flowering Plants

Anatomy of flowering plants deals with the study of internal structure of various organs of flowering plants.

The tyloses are the structures present in the woody tissues of dicot stems. These are the extension of xylem parenchyma cells into the vessel elements(NEET-II 2016).

Cortex tissue is present in between the epidermis and stele. It is multilayered and is made up of parenchymatous cells with big intercellular spaces (NEET-II 2016).

Wood of gymnosperms does not contain vessels.e.g- Pinus is vesseless. Vessels are the characteristic feature of angiosperms (AMU 2015).

Protoxylem is the major anatomical structural difference which distinguishes dicot stem from dicot root.In dicot stem, the protoxylem is present towards the center (Pith) and metaxylem is present towards the periphery of the organ. This type of xylem is known as Endarch. In dicot root, the protoxylem is present towards the center. This condition is called Exarch (EAMCET 2015).

Springwoods contains vessels with a wider lumen and are formed in the spring season. It is light in color and of lower density. Autumn wood contains vessels with the narrow lumen and formed in the autumn season. It is light in color and higher density (Kerala CEE 2014).

Vascular bundles of monocot stem are conjoint, closed and phloem parenchyma is absent in them. Therefore, no secondary growth is observed in monocot stem, except for few exceptions (Kerala CEE 2014).

Endodermis is the innermost layer of cortex, where tangential and radial walls of some cells are deposited with suberin (a waxy material) It makes these cells impervious to the water (Kerala CEE 2014).

The presence of polyarch xylem and the Casparian strip is the feature of the root. So, it is not found in the stem(Kerala CEE 2014).

The cell wall is made up of cellulose and hemicellulose (J&K CEE 2014).

Sieve tubes are component of phloem. It is devoid of the nucleus at maturity and its metabolic activities are controlled by the nucleus of companion cells. Albuminous cells are not produced by angiosperms. They are present in gymnosperms only. In dicot roots, the vascular cambium formed during secondary growth. Primary meristem contributes to the formation of primary plant body (EAMCET 2014).

Dicot Stems – Anatomy of flowering plants

In dicot stems, the cells of cambium present between primary xylem and primary phloem are the intrafascicular cambium. The cells of medullary rays, adjoining this intrafascicular cambium become meristematic and form the interfascicular cambium (NEET 2013).

In dicot stem, the central part is pith. Protoxylem is towards the center, then come the metaxylem, cambium, phloem, pericycle, endodermis, parenchyma, collenchyma and then lastly epidermis (J&K CEE 2014).

Aerenchyma is a parenchyma, which contains wide air spaces that help to make the plant light and buoyant. It is a characteristic of hydrophytes (OJEE 2013).

Vessels are composed of a row of cells placed one above the other. They form a pipe-like structure due to the absence of the septa walls between the cells (BCECE 2012).

Stems are charecterised by endarch condition of xylem, while toots are of exarch condition (BCECE 2012).

Ray parenchyma cells are responsible for lateral movement of water and food materials (AMU 2012).

All the tissue except epidermis and vascular bundles constitute the ground tissue of fundamental tissue. It consists of simple tissues, such as parenchyma, collenchyma, and sclerenchyma. Ground tissue includes cortex, pericycle, medullary rays. In leaves, the ground tissue consists of mesophyll (CBSE AIPMT 2011).

Root is that part of plant body which grows and down into the earth. The primary roots develop from radicle and give secondary and tertiary roots. Lateral roots develop endogenously, i.e. From pericycle between two protoxylems (UP CPMT 2011).

Secondary growth this the growth in girth of stem and roots. Anomalous or abnormal secondary growth is found in some monocot stems such as Yucca, Dracaena, Aloe, Agave, etc (UP CPMT 2011).

Dendrochronology is the determination of the age of a tree by counting annual rings (These rings are formed by the activity of cambium in dicot root and stem) (UP CPMT 2011).

The bulliform cells in the leaves absorb water and they become turgid. Thus, the leaf surface is exposed. When they are flaccid due to water stress, they make the leaves curl inwards to minimize water loss (Kerela CEE 2011).

Many organs of aquatic plants float in water. The mesophyll between upper and lower epidermis is differentiated into palisade parenchyma and spongy parenchyma. Intercellular space is the present amount the spongy parenchyma cells (KCET 2011).

The vascular bundles containing cambium are said to be open, but if no cambium develops, they are referred as closed. In most Monocotyledones, closed vascular bundles are found (DUMET 2011).

Springwood plus autumn wood of a year constitute the annual ring. The spring wood (also called early wood) is light in color and constitutes the major part of the annual ring. The autumn wood (also called late wood) is darker in color (DUMET 2011).

Jute fibers occur in long wedge-shaped bundles outside the xylem (i.e. phloem fibers). These fibers are called as bast fibers (OJEE 2011).

The stele is composed of pith and vascular bundle. On the basis of the orientation of xylem and phloem the vascular bundles of stele are classified as collateral, bi-collateral and concentric. Pith is formed of large parenchymatous cells (OJEE 2011).

Cotton is chiefly composed of epidermal hairs of seed and chemically it is cellulose (OJEE 2011).

When phloem is surrounded by xylem on all sides, such concentric vascular bundle is called amphivasal or leptocentric. Such vascular bundle is found in Dracaena, Yucca, Aloe, etc (J&K CET 2011).

Intercalary meristems are present in apical meristem only. These are separated from the apex during the growth of axis and formation of permanent tissues (CBSE AIPMT 2010).

Sclerenchyma is considered thick-walled lignified supportive tissue characterized by the absence of living protoplast. Their principal function is to provide mechanical support (CBSE AIPMT 2010).

Companion cells are the characteristic element of phloem tissue associated with the sieve tubes in the angiosperms. They are absent in pteridophytes and gymnosperms (Kerala CEE 2010).

Lenticels are small pores formed on the surface of stems and roots of higher plants. Lenticels usually arise below the stomata of the original epidermis, where loose packing tissue becomes waterproof with suberin, leaving large intercellular spaces through which gas exchange can take place (Kerala CEE 2010).

Sieve plates are formed by two adjoining end walls of neighboring sieve elements of sieve tube of phloem. A maturity, these become impregnated with callose pad, which may be seasonal callose (for the only limited unfavorable period) or definitive callose (formed permanently in functionless old sieve tubes) (Kerala CEE 2009).

The correct sequence of structures in order of their location from the periphery to center in a dicot plant. Trichoblasts → Collocytes → Fusiform cells → Tyloses (EAMCET 2009).

The tissue derived from the procambium is called the vascular or fascicular tissue system. It consists of varying number of strands or bundles called the vascular bundles. Each vascular bundle is made up of xylem and phloem necessarily, but the cambium may or may not be present  (WB JEE 2009).

Endodermis generally forms a sheath of the single layer around the vascular region (stele). Presence of Casparian strip in the characteristic feature of root endodermal of cells. The Casparian strip is found on radial and transverse wall of cells (AMU 2009).

Desert grasses often roll their leaves due to the presence of bulliform cells. These are big-sized, thin-walled and large vacuolated cells that frequently occur towards the lower epidermis (AMU 2009).

Companion cells are present between sieve tubes in the phloem of angiosperms. These are the living cells with the large nucleus that controls the activity of non-nucleated sieve tubes (BHU 2009).

Heartwood or  Duramen presents the ventral wood of the plant. It is dark in color and heavier in weight. Living cells are absent. It represents non-functional part of the secondary xylem (wood) (Manipal 2009).

Schmidt (1924) proposed the tunica-corpus theory, which states the presence of two distinct zones in angiospermic shoot apices. The peripheral zone is called tunica and the inner core of cells in called corpus (HPMT 2009).

Apical meristem is present at the apices of shoots and roots of the plant and possesses actively dividing cells. These meristems are virus-free and are responsible for the increase in length and all primary tissues of the plant body originate from them (HPMT 2009).

Vessels or tracheae are made up of a row of cells, placed one above the other, with their intervening walls absent or variously pored. The walls of vessels are lignified and hard, but not very thick. The cell cavity or the lumen is wide. The thickening may be annular, spiral, scalariform, reticulate and pitted (CBSE AIPMT 2009).

In dicotyledonous stem (e.g. Cucurbita), the condition of xylem is endarch as the metaxylem away from the center and protoxylem towards the center (CBSE AIPMT 2009).

The vascular bundles in Hordeum vulgare (barely), are scattered in ground tissues, many in number and vary in size venter of the ground tissue, oval or rounded in outline, conjoin, collateral and closed (CBSE AIPMT 2009).

Sorghum (family – Poaceae) is a monocot plant. The leaves of monocot do not contain palisade parenchyma because the mesophyll of monocot leaf is not differentiated into palisade and spongy parenchyma, all being thin-walled, chlorophyllous and irregularly compactly arranged with fewer intercellular spaces (CBSE AIPMT 2009).

Plerome is the central histogen which form stele or part of stem and root inner to endodermis. Part of plerome that forms vascular tissues is called procambium (CBSE AIPMT 2008).

Intercalary meristem is present away from the apical meristem in primary permanent tissue. It is present at the base of internodes, e.g. in the family – Gramineae or at the base of leaves, e.g. Pinus or at the base of the node, i.e. Mentha. Intercalary meristem is responsible for the increase in the length of different internodes in sugarcane (CBSE AIPMT 2008).

Tylose is balloon-like structures develop fro the xylem parenchyma and it block the passage of xylem vessels and ray parenchyma. So, also called tracheal plug. These tyloses block the passage of xylem vessels and ray parenchyma (BHU 2008).

Tracheids are the most primitive type of conducting elements in xylem. The xylem of gymnosperms consists of tracheids only.Companion cells are thin-walled elongated cells in the phloem. They are living and contain dense protoplasm with the large elongated nucleus. According to histogen theory, periblem is the middle dermatogen, which gives rise to the cortex of root and stem (Keral CEE 2008).

In roots, endodermis is the innermost layer of cortex. Some of the thin-walled endodermal cells present opposite to the xylem patches are called passage cells or transfusion cells. Passage cells help in the transfer of water and dissolved salts from the cortex directly into the xylem and ultimately to the pericycle (CBSE AIPMT 2007).

Aerenchyma cells are parenchymatous or thin-walled cells with air cavities. They are well-developed in hydrophytes (e.g. Hydrilla, Vallisneria, Potamogeton, etc.), These help in buoyancy or floating (BHU 2007).

Depending upon the number of protoxylem elements, the roots are the monarch (having single protoxylem group), diarch, Triarch, tetrarch, pentarch, hexarch, and polyarch. The dicot roots show mono to hexarch condition (UP CPMT 2007).

Endodermis (also called starch sheath) is the innermost layer of cortex. It is the uniform uniseriate layer of tightly packed barrel-shaped (vertically elongated) cells. Casparian endodermal cells. Endodermis functions as biological check post. It helps in preventing loss of water from stele (UP CPMT 2007).

Apical and intercalary meristems always increase in the height of the plant and lateral meristem, it is responsible for secondary growth (increase in girth), but secondary growth doe not occur in monocots, e.g. maize (EAMCET 2009).

In collateral vascular bundles, phloem is situated towards outer side and xylem towards the inner side and both are found on same radii, but in monocot stem, vascular bundles are closed, i.e. cambium is absent (AIIMS 2008).

Quiescent center

The quiescent center is present in the center of the root apex and the cell division is very few in the quiescent center. The quiescent center may function as reserve meristem (AIIMS 2007).

Morphology of Flowering Plants

Anatomy of Flowering Plants