The Plant Organs

The Root

Position: The lower part of the plant axis.

Functions: Anchorage and absorption of water and minerals.

The epidermis: Thin walled, without cuticle, having root hairs (piliferous layer), uniseriate or multiseriate (velamen) in air roots that act as absorptive tissue.

The cortex: Parenchyma, no chlorophyll except in some hydrophytes and aerial roots of epiphytes, act as storage tissue.

The exodermis: The outer subepidermal layer of the cortex, suberized or lignified, single layer or many layers, act as protective tissue.

The endodermis: The inner boundary of the root cortex, with casperian strip and passage cells, regulates the water passage to xylem.

The vascular cylinder: Surrounded by the pericycle that is the origin of the lateral roots, the vascular bundle of radial type, the protoxylem exarch.

Monocot roots                                                 

The xylem is polyarch; more than 17 arches.  

Dicot roots

The xylem is limited; from 2-12 arches.

The Stem

The epidermis: The outer layer of the stem, fiberous epidermis may be present in monoct stems, trichomes may be present.

The cortex: Between the epidermis and the vascular cylinder, of parenchyma, chlorenchyma, collenchyma or fibers, in monocot stems the ground tissue is undifferentiated into cortex nor pith.

The starch sheath: The innermost layer of the cortex, containing starch grains.

The primary vascular system: External phloem and internal xylem (conjoint vascular bundles), the protoxylem endarch.

The pith: Parenchyma with leucoplastids, may contain crystals, tannins or sclereids.

Monocot stems                                                                  

The vascular bundles are:

numerous, scattered, collateral, closed, xylem vessels in the form of V or Y, presence of xylem cavity, presence of fiber sheath around the bundle.

Dicot stems                                                                  

The vascular bundles are:

few, arranged, collateral or bicollateral, opened, xylem vessels in the form of rows, absence of fiber sheath .

Root-Stem Transition

       The root and the stem make a continuous structure called the axis of the plant. The vascular bundles are continuous from the root to the stem, but the arrangement of vascular bundles is quite different in the two organs; the stems possess collateral bundles with endarch xylem, whereas the roots possess radial bundles with exarch xylem. The change of position involving inversion and twisting of xylem strands from exarch to endarch type is referred to as vascular transition, and the part of the axis where these changes occur is called transition region.

Type A: In Fumaria, Mirabilis and Dipsacus, and others, each xylem strand of the root divides by radial division forming branches, they swing in their lateral direction; one towards right and the other goes to the left. These branches join the phloem strands on the inside. The phloem strands, do not change their position and also remain unchanged in their orientation. They remain in the form of straight strands continuously from the root into the stem. In this type as many primary bundles are formed in the stem as many phloem strands are formed in the root.

Type B: In Cucurbita, Phaseolus, Acer and Trapaeolum and others, the xylem and phloem strands fork, the branches of the strands of both swing in lateral direction and join in pairs. After joining in the pairs they remain in the alternate position of the strands in the root. The xylem strands become inverted in their position and the phloem strands do not change their orientation. This way, in the stem, the number of bundles becomes double of the phloem strands found in the root. This type of transition is more commonly found.

Type C: In Lathyrus, Medicago and Phoenix, the xylem strands do not fork and continue their direct course into the stem. These strands twist through 180 degrees. The phloem strands divide soon and the resulting halves swing in the lateral direction to the xylem positions. The phloem strands join the xylem strands on the outside. In this type as many bundles are formed as there are phloem strands in the root (as in type A).

Type D: This type is rarely found and is known in only a few monocotyledons (e.g., Anemarrhena). In this type half of the xylem strands fork and the branches swing in their lateral direction to join the other undivided strands of xylem which become inverted. The phloem strands do not divide but they become united in pairs. These united phloem strands unite with the triple strands of the xylem. Thus, a single bundle of the stem consists of five united strands. In this type half as many bundles are formed in the stem as there are phloem strands in the root.

The Leaf

The epidermis: contains stomata and trichomes, upper (adaxial) and lower (abaxial).

The ground tissue: the ground tissue of the leaf is called mesophyll.

Types of leaves:

Dorsiventral:

Palisade below the upper epidermis, the spongy at abaxial surface.

Isolateral:

Palisade is found on both ad- and abaxial surfaces, one row at each side.

Isobilateral:

Palisade found on both sides, two rows at each.

The vascular system:

The vascular bundles are called veins, the arrangement of veins is called venation, the phloem is directed towards the lower epidermis.

Monocot leaves

The blade is undifferentiated into midrib nor wings.

The ground tissue is undifferentiated into palisade nor spongy.

Vascular bundles are numerous.

Mechanical tissue of sclerenchyma beneath epidermis.

Dicot leaves

The blade is differentiated into midrib and wings.

The ground tissue is differentiated into palisade and spongy.

Vascular bundles are one median and numerous laterals.

Mechanical tissue of collenchyma beneath epidermis in midrib region.