From the very beginning trees have a very important role in our Hindu scriptures. Some trees have been described on and off in our scriptures. Some type of leaves are used to keep eatables (plantain leaves in South), some leaves are to be consumed themselves (Cilantro, Mint, Maithee); some leaves are used for smell as well as eating (Cilantro, Mint, Maithee). Some leaves are cooked and eat (Spinach, Maithee, Mustard). Some are used for mere smell alone (Curry leaves). Let us see what kind of leaves are popular and used in our scriptures.

There are five leaves may be offered to Devtaa during their Poojaa. These five leaves represent the five great cosmic elements - Water, Air, Earth, Fire, Ether. All Devtaa have their own nature also that is why a certain kind of leaf is liked by a certain Devtaa, that is why a certain kind of leaf is to be offered to that specific Devtaa. Devtaa are in the forms of light. The principles of Devtaa are activated with the help of these five elements. These elements are quickly activated through these leaves, for example Tulasee leaf to Vishnu and Krishn and Raam; Bel leaf to Shiv; Doorvaa grass is for Ganapati.

In Karm Kaand all - Tulasee, Bel and Doorvaa grass are mandatory. Ritual worship is incomplete without the leaves. The leaves of five trees - Peepal (Ficus religiosa), Audumbar (glomerous fig tree), Ashok (Jonesia Asoka), mango, and Vat (ficus indica) are of special spiritual significance in religious rituals. They are known as Panch Pallav (five leaves)

--In Bhagavad Geetaa, Krishn says - "If anybody offers me even a leaf with full devotion, I accept it by manifesting in gross form."
--Everybody knows that Shiv loves Bel (wood apple) leaves.

In addition to offering to Devtaa, there are some other leaves used in worshipping processes, for example in decorating the worshipping place. Here we give the sacred leaves which are very important in worship --

Tulasee Leaf
Read about Tulasee in detail here

Tulasee is a sacred plant. Sins of even many lives are destroyed just by seeing, touching, meditating, offering obeisance, planting and using of Tulasee. All deities reside in the plant of Tualsee from its roots to the tip. It is said that whoever has a Tulasee plant in his house makes nine miles (one Yojan) area as pure as River Gangaa. There is a specialty of this plant that its every part is fragrant - from roots, wood, leaves, flowers, to seeds... Even the soil in which it is grown becomes fragrant with its fragrance.

--Skand Puraan says that some of the drops of Amrit (nectar) fell on the ground during the process of Samudra Manthan. Tulasee was formed out of those drops. Brahmaa Jee handed over them to Vishnu. It also says that "Tulasee ye Vichinvant Dhanyaaste Kar Pallavaah" - means they are blessed who pluck Tulasee leaves with their hands for worship."
--Padm Puraan says that offering flowers made of gold, gems, pearls etc are not even 1/16th in comparison to a Tulasee leaf.

Although the whole Tulasee tree or plant is very important for a Hindu but Tulasee leaf is especially important leaf in a Hindu's life. The Vaishnav cannot live without Tulasee plant, because he will need Tualsee leaf twice a day for the Naivedya offered to Vishnu.
--Tulasee leaves garland is offered to Badaree Naath Jee and people get it in Prasaad.

Do not pluck Tulasee leaves singly, pluck in pairs
Pluck the Tulasee leaf without shaking the plant
When you pluck its flower (Manjaree, or inflorescence) it should be with a Tulasee leaf.
Read  Tulasee to know more about Tulasee.

Bel Leaves (Bilva Patra)
Another important leaf is of Bel (wood apple) leaf or Bilva Patra. This leaf is very dear to Shiv and Shiv is very happy with Bel leaf (including Bel fruit and Dhatooraa fruit). As Vishnu's Naivedya is incomplete without Tulasee, Shiv's worship is incomplete without Bel leaves. Mostly all Shiv temples have one Bel tree on its premises.
Never ever offer Tulasee leaf to Shiv Jee as Tulasee is supposed to be the wife of Vishnu, Shiv cannot accept it.

Doorvaa or Darbhaa Grass
This is not a leaf in real sense, it is a grass technically, still it is a kind of leaf. Without this no worship is complete. Most Pandit (priests) keep it with them in cases it is not available with Yajamaan. It is mostly used to sprinkle water over the Devtaa, tie it in hands etc.

Betel Leaves
See also   Betel Leaf
Betel leaf (Paan kaa Pattaa) is also very sacred and is used in worship. Betel leaves are used to put inside the water Kalash and sprinkle water on Poojaa materials and Devtaa. It is also offered to Devtaa also as a food because it increases love and relationship between Devtaa and the worshipper. In fact, a monk who has renounced all worldly relationship is forbidden for taking betel leaves because he should not develop worldly attachment or bondage. Betel leaf should never be heated for eating purpose, it is eaten fresh (although its one medicinal use is by heating it and keeping on the place). It has many other uses also.

It is customary to exchange Betel leaves on important occasions, such as on marriage occasion husband and wife feed Paan to one another as a sign of love and trust. It is because every betel leaf has an innate quality that we may not be aware of, for example --
--There is a herb known as Paataal Bhairavee which can nurture enmity between two individuals.
--There are herbs that can transform iron into gold.
--Some herbs can bestow wealth.
--We even find the references of herbs that can make a man disappear.
Likewise, betel leaves possess the unique property of nurturing love between two people.

Mango Leaves
Mango leaves are considered very auspicious. They are used to keep on water Kalash for decoration. On all auspicious occasions, like marriage, birth of a child etc and worship times, mango leaves are hung on the doors tied in a string and their twigs are used to decorate the Poojaa place.
Bigger grown up leaves were used by people in yester years for cleaning teeth and tongue in Kerala. If at all a small bit goes inside or extract go inside with saliva while cleaning, it does not cause any harm.

Banana Leaves
The whole banana tree is very holy. Normally all temples have a banana tree on their premises. People worship banana tree on Thursdays. Its leaves with a short trunk are used to decorate Poojaa place.

Neem Leaves
Neem leaves are not sacred leaves as other leaves are mentioned above, but they have great medicinal value and are used to ward off evil effects of spirits. It is great air purifier. Its tender sticks are used to clean teeth (as Daantun or Dataun). In the case of Chickenpox disease, a twig is kept near the head of the patient. This can be changed as the need be.

Leaf in Dwaapar Yug
Even in Dwaapar Yug there is a reference of a Cheeraa leaf bit which was stuck to the Akshaya Paatra of Draupadee. It was taken out and consumed by Lord Krishn when Durvaasaa Rishi along with his 10,000 disciples wanted to eat food at Yudhishthir place while they were in exile.

It is said that Krishn left Duryodhan's good food and ate leaves Saag at Vidur's place when He went as a Peace Messenger to Hastinaapur.