Anatomy of Flowering Plants Test - 2

In exarch condition of xylem, protoxylem or the first formed xylem is present towards the periphery while metaxylem or later formed xylem is present towards the centre of the root. Such an arrangement can be seen in roots.

Endodermis is the innermost layer of cortex that consistws of tightly packed barrel shaped cells. It is called starch sheath in case of dicot stems. Radial and and tangential walls of endodermal cells possess thickenings of lignin, suberin and cutin in the form of strips or bands, which are known as casparian bands or casparian strips.

vascular bundles in monocot roots are radial, polyarch and exarch. Large number (more than 6) of xylem and phloem groups alternate with each other. A well devloped pith is present in monocot root. 

In a dicot stem (e.g., In sunflower), hypodermis is made up of collenchyma, which may be green. In monocot stem  (e.g., maize), hypodermis is formed of non green sderenchyma tissue.

Ground tissue of a monocot stem is a mass of similar cells which does not show distinction into cortex, endodermis, pericycle, pith, etc.

A monocot stem lacks secondary growth. The vascular bundles are oval or rounded in outline. They contain both phloem and xylem. Phloem lies towards the outside and the xylem on the inner side. Cambium is absent as the whole procambium is consumed in the formation of vascular tissues. Xylem is in the form of letter Y. It is endarch, i.e., protoxylem lies towards the centre of the stem. Xylem is made up of vessels, tracheids, xylem parenchyma and a few xylem fibres. Metandem generally consists of two large oval or rounded vessels lying at the upper two angles of xylem. Protoxylem cavity is present at the end of protoxylem vessels. 

In a dicot stem, a well developed pith (made of parenchymatous or occasionally sclerenchymatous cells) is present whereas in a monocot stem, pith is absent. In a dicot root, pith is poorly developed whereas in a monocot root, a well developed pith is present.

Dorsiventral (bifacial) leaves are commonly horizontal in orientation with distinct upper (adaxial) and lower (abaxial) surfaces. Mesophyll is distinguishable into palisade and spongy tissues with palisade usually restricted to the upper (adaxial) side. Vascular bundles are conjoint and collateral i.e., they possess both xylem and phloem which lie on the same radius. Xylem lies towards the upper (adaxial) side of leaf while phloem lies towards the lower (abaxial) side of leaf. Most of the dicotyledonous leaves are dorsiventral.

Based on position of protoxylem in relation to metaxylem, the xylem may be exarch/centripetal, endarch/centrifugal, mesarch and centerarch. In endarch condition, protoxylem lies on the inner side of metaxylem e.g., dicot and monocot stems.

In an isobilateral (equifacial) leaf, vascular bundles are conjoint and collateral. Xylem lies towards the adaxial (upper) side and phloem lies towards abxialal (lower) side of leaf.

In dicot leaf, bundle sheath is generally single layered and formed of colourless cells. Bundle sheath extensions are parenchymatous. In monocot leaf, bundle sheath may be single or double layered and the cells generally possess chloroplasts. Bundle sheath extensions are sderenchymatous.

Word 'stele' is taken from Greek language, which means 'pillar'. Stele consists of pericycle, vascular bundles (xylem and phloem) and pith (if present).

In dicot leaves, stomata are generally present on lower epiderm is (hypostomatic), whereas in monocot leaves, they are present on both the surfaces (amphistomatic). In free -floating hydrophytes, stomata are restricted to upper epidermis (epistomatic) whereas in submerged hydrophytes, stomata are either non-functional or ab sent. In algae and fungi, stomata are totally absent. Stomata are sunken (deepseated) in case of xerophytes.

Bicollateral va scular bundles have phloem both on the outer and inner side of xylem. All the three lie on the same radius. Usually a strip of vascular cambium is present on both outer and inner sides of xylem. Bicollateral vascular bundles occur in Cucurbitaceae (e.g., Pumpkin or Cucurbita pepo, Ridge gourd or Luffa cylindrica) and some members of families Solanaceae, Convolvulaceae, etc.

In monocot root, a large number of vascular bundles are arranged in the form of a ring around the central pith.
Due to the presence of casparian strips, the endodermal cells do not allow wall to wall movement of substances between cortex and pericyde, in a primary dicot root.

In most monocot stems, a sclerenchymatous bundle sheath is generally present on the outside of each vascular bundle.

There is no such distinction between a young and an old root of monocotyledonous plant. This is due to the absence of secondary growth in the monocot roots. The centre of monocot root is occupied by the pith. It consists of parenchymatous cells which may be rounded or angular. Intercellular spaces are present amongst the pith cells. The pith cells store food. Xylem and phloem bundles are numerous and are 8 or more in number. Xylem vessels are oval or rounded.

The epidermis of dicot stem bears several unbranched multicellular hair or trichomes. The hypodermis is made of 3−4 layered sub-epidermal collenchyma tissue. Vascular strand is in the form of eustele or a ring of vascular bundles present around the central pith and inner to the pericycle.

The term stele was coined by Van Tiegham and Dauliot (1886). It is the axial portion of plant axis. Anatomically, all the tissues on the innerside of endodermis such as pericyde, vascular bundles and pith constitute the stele. Eustele is the type of stele in which a ring of vascular bundles is present around the central pith and inner to the pericyde e.g., dicot stem. Stele containing irregularly scattered vascular bundles is called atactostele, e.g., monocot stem. Pteridophytes are the first plants possenssing stele.

There is a water contain ing lysigenous cavity in monocot stem, at the end of protoxylem vessels. It is formed by the dissolution of inner protoxylem vessel and parenchyma.