Angiosperms

Angiosperm are flowering plants that are classified based on characteristics that include (but are not limited to) cotyledon structure, pollen grains, as well as flower and vascular tissue arrangement. These have special structures flowers and fruits which were absent in previous divisions. These are known as flowering plants (Fig.1).

Flowers:

It is a structure that is needed for sexual reproduction. It has four parts:
1. Calyx- This is a whorl formed by green leaf-like structures known as sepals. This is the outermost part of the flower.
2. Corolla- This is present inner to the calyx and is formed of coloured leaf-like structures known as petals. The colour helps to attract pollinators.
3. Carpel (Gynoecium)- These are megasporophylls that is, produce female gamete, egg. It is formed of stigma, style and ovary. Stigma is a flat receptor to receive the pollens that come from pollination. The ovary is an oval structure that contains ovule/ovules which develop into seeds. Style is a thin structure that connects stigma with the ovary. The carpel is also known as the pistil. There can be single or multiple carpels in flower.
4. Stamens (Androceium)- These are microsporophylls i.e. produce male gamete, pollen grains. It is formed of thin stalk-like structures known as filaments and a sac known as anther. The haploid pollen grains are formed in the anther.

Fruits:
After fertilisation, the ovule develops into a seed and the ovary develops into a fruit.

Sexual reproduction in angiosperms.

The angiosperms have megasporangium and microsporangium in flower. The megasporangium is present in the carpel which has the female gametophyte (embryo sac). It contains egg. The microsporangium produces pollen grains that contain sperms. When the pollen grain land on the stigma by pollination, a pollen tube is formed. The sperms pass through it to reach the egg and fertilise. This forms the zygote which develops into an embryo. The ovule develops into a seed in which embryo is present. When it reaches the ground, it grows to form a new plant.

The angiosperms have a single phylum known as anthrophyta which is divided into three groups: basal angiosperms, monocots and eudicots.

1. Basal angiosperms- These are tall trees with undifferentiated petals and sepals. A receptacle is present that is cone-shaped. It has stamens at the bottom and capel at the top. Example: Magnolia, water lily, and avocado tree.
2. Monocots- These have only one cotyledon in their seeds. Example: Maize, corn and wheat.
3. Eudicots- These have two cotyledons. More than 2/3rd of the angiosperm belong to this group. Example: Oak, maple and peaches.

Organization and Structure of Angiosperms

Angiosperms are also known as the flowering plants. These plants are made up of two systems: the root system and the shoot system.

Root system

It consists of roots with its branches. The main part of the root extending from the radicle is known as the primary root. From the primary root, lateral roots are formed, which are known as secondary roots. From the secondary root, tertiary roots originate.

The root system is of three types (Fig. 1):

1. Tap root system: It is formed of the primary roots and branches. It is seen in dicot plants. Example: Banyan, Mustard seed, and Mango.  
2. Fibrous root system: It is formed of the large number of root branches originating from the base of the stem or after replacement of the primary root. It is found in monocot plants. Example: Wheat, Onion and Grass. 
3. Adventitious root system: These are the roots that are originated from the plant parts other than the radicle. These help in functions like vegetative propagation and providing mechanical support. Example: Banyan tree and Oak trees.

Functions of the root system:

1. Absorption of water and minerals.
2. Providing anchorage to the plant.
3. Storage of food reserves.
4. Synthesis of plant hormones.

Regions of the root

The root is divided into four regions (Fig. 2):

1. Root cap: It is a cup-shaped cap that protects the tip of the root. It is formed of loosely arranged parenchyma cells. The cells of the root cap can sense the light and gravity due to which they grow in the soil.
2. Region/area of meristematic activity: This is present just above the root cap. In this region, active cell division occurs. It is formed of parenchyma cells with dense cytoplasm without a vacuole.
3. Region/area of elongation: This is present above the region of meristematic activity. In this region, the newly divided cells elongate to help in root lengthening. The vacuoles of the cell fill and swell by the water. This results in the elongation of the cells.
4. Region/area of maturation: This region contains mature and differentiated cells. Root hairs originate from the epidermal cells of this region.

Remember: A quiescent center is present between the root cap and the actively dividing cells. The cells at this center divide at a slower rate.

Root Modifications

In some plants, the roots modify to perform functions other than their normal function, that is, absorption of water and minerals.

1. Storage root: These roots are modified to store food. Example: Turnip and Carrot. These are of different types, such as conical, fusiform, napiform, and tuberous roots.

i. Conical- It is of cone-shaped with a broad base and narrow or pointed apex. Example: Carrot (Fig. 3a).
ii. Fusiform- It is of spindle-like shape, i.e. swollen in the center tapered towards the ends. Example: Radish (Fig.).
iii. Napiform- It is swollen or spherical at the base and tapered at one end. Example: Turnip (Fig. 3c).
iv. Tuberous- It is swollen and fleshy with an irregular shape. Example: 4 O’clock plant (Fig. 3d).

2. Prop roots: These are roots originating from aerial stem branches hanging towards the ground. These help to provide support to heavy plant branches. Example: Banyan tree (Fig.4).

3. Stilt root: These are the roots coming out from the lower nodes of the plant and grow obliquely towards the ground. They provide mechanical support. Example: Sugarcane and Maize
4. Pneumatophores or respiratory roots: These roots are present in the halophytes plants and help in respiration. These roots originate inside the ground, extend outwards and then from vertically upwards. These help the plants to get sufficient oxygen. Example: Rhizophora and Avicennia (Fig.4).

5. Nodulated roots: It is seen in secondary roots as small swellings. These nodules provide shelter to the nitrogen-fixing bacteria, such as Rhizobium. Example: Legumes.
6. Climbing roots: These originate from the nodes and help to climb. Example: Money plant and Betel.
7. Assimilatory roots: These are hanging roots that contain chloroplasts and perform photosynthesis. Example: Water chestnut plant and Taeniophyllum.
8. Epiphytic roots: These roots are found on branches of large trees to get sunlight. They have a tissue known as velamen that absorbs moisture and helps in photosynthesis. Example: Orchids.
9. Sucking roots or Haustorium: These are microscopic roots that originate from the parasitic plant and penetrate into the vascular tissues of the host plant. These help in the absorption of water and minerals. Example: Cuscuta.

Shoot system

The part of the plant present above the ground is known as the shoot system. It includes the stem on which leaves, flowers and fruits are present. Leaves originate from the notes of the stem, and the distance between two nodes is known as internodes.

Stem
The stem is green in color during primary growth and becomes brown and woody later after secondary growth. It develops from the plumule of the embryo. It grows against the gravity towards the sunlight. Multicellular hairs or Trichomes are found on the stem.

Modifications of the stem

Fig 6: Modifications of the stem
Image source: Openstax

A. Underground modifications of the stem:

1. Rhizome: It is a fleshy modification of stem and is not green in color. It contains nodes and internode. From the nodes, dry scale leaves originate that contain axillary buds. Example: Ginger and Alocasia (Fig. 6a).

2. Bulb: It is a stem on which fleshy leaves are present in a condensed arrangement. The adventitious root is also found at the base. Example: Onion (Fig.6b ).

3. Corm: It is similar to rhizome but is relatively more solid. Adventitious roots are either present at the base or on all its body. Example: Colocasia (Fig.6c).

4. Tuber: It is a fleshy thick stem modification covered by a corny skin. It has nodes in the form of depressions known as the eye. From the eyes, buds originate. Adventitious roots are not present (Fig 6d). 

B. Subaerial modifications of the stem
In these modifications, some part of the stem is present underground while some part above the ground.

1. Runners: These stem modifications creep horizontally on the ground. They have long internodes.  From the nodes, axillary bud, leaves, and adventitious roots originate. Some parts of the runner break and give rise to a new plant. Example: Lawn grass and Strawberry plant (Fig 7).
2. Sucker: This stem modification originate from the underground part of the main stem, grows horizontally on the ground for a small distance and then grow obliquely upwards. Leaves and adventitious roots originate from the nodes and then they break off from the mother plant and grow like a new plant. Example: Crysanthemum and Banana plant (Fig 7).
3. Stolon: It originates from the base of the main stem, grows laterally upwards, and then back towards the ground. When it touches the ground, it gives rise to a new stem and adventitious root. It is weak than a sucker. Example: Colocasia and Jasmine (Fig 7).
4. Offset: It is a short form of the runner with the length of one internode. From it, rosette leaves originate above the ground and adventitious roots underground. Example: Water lettuce and Water hyacinth (Fig 7).

Aerial modifications of the stem:

1. Tendrils: These are green and weak stems that help in climbing. These are leafless. Example: Cucurbita and Grapevine (Fig. 18a).
2. Thorns: These are the stem modifications into pointed, hard, and pin-like structures. These help in reducing the transpiration and also help in defense. Example: Bougainvillea and Pomegranate (Fig 18b).
3. Phylloclades:  These are green stem modifications that look flat or cylindrical. These are present in the plants that live in dry habitat and help in reducing transpiration. They also help in photosynthesis. Example: Opuntia and Euphorbia
4. Cladodes: These are one internode long green stem that helps in photosynthesis. The leaves are converted into spines and scales. These help in reducing transpiration. Example: Asparagus.

Leaf

It is a flat green structure that originates from the shoot apical meristem. It contains a bud at the axil which develops into the stem (Fig. 19).

Parts of leaf:
1. Leaf base: It is the part at which the leaf is attached to the stem. Two small lateral leaves known as a stipule, may originate from here.
2. Petiole: It is the part of the leaf that holds and connects the leaf blade to the base.
3. Lamina: It is the flat leaf blade that has veins and veinlets. The middle vein is known as a midrib.

QUIZ: TEST YOURSELF