Leaves on the stem are modified into spine in green succulent plant is called as thorns.

Insects, birds, animals, wind and water are the agents of pollination.

The process by which pollen grains reach stigma is called as pollination.

The important parts of flower is,
(i) Androecium (male reproductive part)
(ii) Gynoecium (female reproductive part)

Two types of reproduction are,
i) Sexual reproduction.
ii) Asexual reproduction.

(i) Plants growing in deserts try to store water. If the surface area is large, more water is lost by evaporation.
(ii) In plants like cactus, the stem is thick and becomes flat like leaves to do photosynthesis.
(iii) The leaves are reduced to small spines with less surface area. This kind of modification is called phylloclade. e.g. cactus.

Many Plants require additional support. Such plants develop roots on their aerial parts to provide mechanical support. These roots grow downward and act as supportive organs. There are three types of modified roots for support.
(i) Prop roots : Roots are modified to provide mechanical support as seen in Banyan tree. These roots grow vertically from horizontal branches of a tree.
(ii) Stilt roots : In sugarcane, and maize adventitious roots arise from the nodes in cluster at the base of the stem. These roots are called stilt roots which gives additional support.
(iii) Climbing roots : In betel and black pepper, nodes or interrnodes bear roots which help in climbing.

In Acacia auriculiformis, petioles expand to form leaf like structure. They carry out the function of leaf (Photosynthesis). This modification is known as phyllode.

Pollen grains are transferred from the anther of one flower to the stigma of another flower of the same kind or different plant is known as cross pollination.

Sub- aerial Modifications : Stem of some plants remains sub- aerial which grow horizontally on the surface of the soil for the purpose of reproduction. There are four types.
(i) Runner: The stem grows laterally on the surface of the soil, breaks up to produce roots where it touches the ground to give rise to new plants. Eg: Centella (Vallarai).
(ii) Stolon: Stolon is a slender branch of the stem that grows upwards to some distance and then bends towards the ground. Upon touching the ground, it gives rise to a new plant. Eg.Wild strawberry.
(iii) Sucker: Sucker is a short and weak lateral branch that grows diagonally upwards and directly gives rise to a new shoot. Eg. Chrysanthemum.
(iv) Offset: An offset is a short and thick branch that arises from the axial part of a leaf. It has thick internodes. It produces a tuft of leaves and cluster of small roots below. Eg: Eichhornia.

In underground modified stem, whole stem is burried under the ground and it has definite growth. Some stems that grow under the ground to store food. There are four types of underground stems they are,
i) Rhizome:
It is underground thick stem with nodes and internodes with scale leaves at the node.
(Eg) Ginger and turmeric.
ii) Corm:
This underground stem is round in shape and flat at the top and bottom. The daughter plants are arise from their buds. (Eg) Colocasia.
iii) Tuber:
It is an enlarged spherical underground stem and stores food. It has many dormant buds and it grows into a new plant. (Eg) Potato.
iv) Bulb:
It is a condensed stem which is disc like and stores food in the fleshy leaves.
The bulb has two types of leaves
1) Fleshy leaves - Inner part of stem
2) Scaly leaves - Upper part of stem
(E.g) Garlic and onion

The process by which pollen grains reach stigma of the flower is called as pollination. Pollination can be classified into two types based on their mode of transmission.
i) Artificial pollination:
The transfer of pollen grains from anther to stigma by manually is called as artificial pollination
ii) Natural Pollination:
1) The transfer of pollen grains from anther to stigma by insect or any other natural sources is called as natural pollination.
2) Pollination can be classified into another two types based on their site of transmission
i) Self pollination:
Pollen grains are transferred from the anther to the stigma of the same flower or to another flower of the same plant is known as self pollination.
ii) Cross pollination:
Pollen grains are transferred from the anther of one flower to the stigma of another flower of the same kind or different plant known as cross pollination.