Phyllotaxy
The leaves remain arranged on the stem in such a manner that they can get the maximum amount of sunlight for their physiological functions.
Phyllotaxy Definition: The arrangement of leaves on stems or branches in a definite manner is known as phyllotaxy.
Phyllotaxy Types:
Phyllotaxy is mainly of three types—
- Alternate or acrylic or spiral,
- Opposite and
- Whorled. The last two types are known as cyclic type.
Alternate or acyclic or spiral: In this type, one leaf develops from each node and remains spirally arranged around the stem. Examples are china rose, and mango. Here, phyllotaxy is determined by passing an imaginary line through the bases of the leaves.
Read and Learn More: WBCHSE Notes for Class 11 Biology
This shows a spiral path is known as a genetic spiral. if this spiral path is placed on a flat surface then it is known as a flat spiral. The distance between the bases of two consecutive leaves is known as divergence.
” phyllotaxy definition”
The angle subtended at the center of the shoot by two consecutive leaves is known as angular divergence. The imaginary vertical line that connects the leaves vertically is known as orthostichies.
On the basis of the number of orthostichies or the number of rows in which the leaves are arranged,
Alternate phyllotaxy is divided into the following types—
- Distichous or 2-ranked phyllotaxy: Leaves are arranged in two rows, i.e., two orthostichies can be drawn. example, sugarcane, maize
- Tristichous or 3-ranked phyllotaxy: Leaves are arranged in three rows, i.e., three orthostichies can be drawn. exampleCyperus sp., orange.
- Pentastichous or 5-ranked phyllotaxy: Leaves are arranged in five rows, i.e., five orthostichies can be drawn. example Hibiscus rosa-sinensis.
- Octastichous or 8-ranked phyllotaxy: Leaves are arranged in eight rows, i.e., eight orthostichies can be drawn. example papaya.
whorled leaf arrangement examples
Opposite: In this type, two leaves develop from each i node opposite to each other.
It may be of two types— Opposite decussate and opposite superposed.
- Opposite decussate: In this type, consecutive pairs of opposite leaves are arranged at a right angle to each other. example Ocimum sanctum, Calotropis procera, etc.
- Opposite superposed: In this type, successive pairs of opposite leaves are arranged in one plane. example Quisqualis sp., Hiptage madablota, etc.
Whorled or verticillate phyllotaxy: In this type, more than two leaves develop from each node forming a whorl. example, Alstonia scholars, etc.
phyllotaxy of leaves
Modifications Of Leaves
Other than photosynthesis and transpiration leaves perform other functions. For that purpose, they modify themselves through different adaptations.
Different types of modifications are discussed below—
Leaf tendrils: In weak plants, the lamina becomes modified either partially or wholly into a coiled thread-like structure called a tendril. This tendril helps the plant to climb up support.
These can be of the following types—
Whole leaf tendril: In this type, the entire leaf gets modified into a tendril. example Lathyrus aphasia.
Leaflet tendril: In this type, the terminal leaflets are modified into tendrils. example Pisum sativum.
“phyllotaxy “
Petiolar tendril: In this type, the petiole is modified into tendrils. example Clematis gouriana.
Leaf apex tendril: In this type, the leaf apex is modified into a tendril. example Gloriosa Superba.
Stipular tendril: In this type, the stipules are modified into tendrils. example Smilax macrophylla.
Leaf spines: The leaf may be completely or partially modified into a spine. This protects the plants from predators and reduces the rate of transpiration in plants.
These are of the following types-
- The apex of the lamina can be modified into spine as found in Phoenix sylvestris.
- The margin of the lamina can be modified into the spine as found in Argemone mexicana.
- The apex and margin of lamina both can be modified into spines as found in Aloe perfoliata.
- Leaves can be modified into spines. But the leaves that emerge from axillary buds remain normal, as found in Berberis vulgaris.
whorled phyllotaxy
Fleshy or succulent leaves: In some plants, the leaves become fleshy due to the storage of water, mucilage, and food. This is mostly found in xerophytes and halophytes. example Bryophyllum sp., Basella rubra, etc.
Leaf or leaflet hooks: In some plants, three-terminal leaflets of a compound leaf get modified into sharp, curved, and anchored on the support for climbing example biononia angiocath.
Root-like structure: In some aquatic plants, the submerged leaves are modified into narrow root-like structures. These adventitious roots absorb water and help to float the plant on water. example Myriophyllum indicum.
Leaf-pitcher: In Nepenthes khasiana, an insectivorous plant, the leaf lamina is modified into a pitcher with a lid developed from the apex. The pitcher helps the plant to fulfill its nitrogen requirement by trapping and digesting insects.
Bladder: A rootless aquatic herb, the bladderwort {utricuiaria stellaris), has segmented leaves. Some of the segments of the leaves are modified to form bladder-like structures. These bladders help them to trap insects and digest them.
Water reservoir leaf: In Dischidia rafflesiana, a non-insectivorous plant, the leaf lamina is modified to form a pitcher-like structure. The main function of this pitcher is to store rainwater for future use.
This plant is an epiphytic climber. Certain adventitious roots develop from the node, from where the stalk of the pitcher develops. These roots absorb water from the pitcher by entering into the cavity of the pitcher.
Scale leaves: These are membranous leaves that mainly provide protection to the plants. example zingiber officinale.
Functions Of Leaf
Leaves serve two types of functions—primary and special functions.
Primary functions: The basic functions of leaves are—
Photosynthesis: Leaves produce carbohydrates through photosynthesis, which is the main source of food.
Gaseous exchange: The gaseous exchange in plants occurs through the stomata present on the leaves.
Transpiration: The stomata also help in transpiration and thus maintain the water balance in the plant body.
Protects bud: Leaves protect the apical and axillary buds during their development.
Transportation: The vascular bundles of the leaves help in the conduction of food and water within the plant body.
Special functions: Leaves of many plants perform some special functions. Those are—
Food and water storage: The fleshy leaves of xerophytes and some other plants store mucilage and food for future use. example Allium cepa, Aloe vera.
Protection: Some leaves are modified into thorns or spines. These structures protect the plants from different external factors. example Argemone spv Cacti sp.
Climbing: In some plants, leaves get wholly or partially modified into tendrils. These tendrils help the weak stem to climb up on supports. example Pisum sativum.
Reproduction: Adventitious roots and vegetative buds emerge from the leaf margin of Bryophyllum sp. These give rise to new plants.
Catching insects: Some plants have specialized leaves through which they can catch insects. Such plants are called insectivorous plants. These plants grow on nitrogen-deficient soil. To overcome this they feed on insects, from which they get nitrogen.
Helps in floating: In aquatic plants, the leaves contain aerenchyma cells, which help the plants to float on water.
The Inflorescence
As the plants mature, flowers grow on the floral axis either singly or in clusters. Flowers are arranged in various ways on the floral axis in different plants.
The Inflorescence Definition: The arrangement of flowers on the floral axis either singly or in clusters, is known as inflorescence.
The stalk of a solitary inflorescence is known as the floral axis or peduncle. Sometimes, it branches out and bears flowers at the branched apices. The stalk of the individual flowers is called pedicel.
A long, simple, or branched peduncle is known as a rachis. The small main axis of a spikelet present in grass-like plants is called rachilla.
The unbranched naked peduncle that develops from the underground stem, is termed as scape. The stalked flowers are called pedicellate flowers, whereas the flowers without stalks are known as sessile flowers.
Some plants bear flattened peduncles. This is known as a receptacle. The part on which the floral parts grow is called the thalamus. The conical receptacle is known as the torus.
Bracts
Sometimes, the flowers grow at axils of expanded leafy structure, known as a bract. The flowers having bract are known as bracteate flowers and those without bract are known as ebracteate flowers.
Sometimes, very small thin bract-like, leafy, or scaly structures develop on the flower stalk in between the flower and bracts. These structures are known as bracteoles or secondary bracts.
Bracts are of various types—
- Leafy or foliaceous,
- Scaly,
- Spathy,
- Petaloid,
- Epicalyx,
- Involucre,
- Glume, and
- Cupule.
Types of Inflorescence: Inflorescence is mainly of three types.
They are as follows—
