In India, chai is more than just a cup of tea to start the day - the thick sweet drink is an integral part of the rhythm of life. Zach Marks and Resham Gellatly have been documenting the culture of Indian chai and the people who sell it - known as chai wallahs.

Image copyright Resham Gellatly

Tea is India's most popular drink - the country consumes 837,000 tonnes of it every year. The ritual of drinking chai transcends all boundaries, and roadsides are dotted with chai wallahs who serve it boiled up with spices, sugar and milk.

Image copyright Zach Marks and Resham Gellatly

Santosh strains a vat of boiling chai at his shop in Mumbai. Since he began selling tea 15 years ago, the area has changed dramatically. Many of the small businesses where Santosh once delivered chai have been replaced by large office buildings which he can't enter. But many people working in the new developments have become regular customers, preferring Santosh's chai made with thick milk and fresh ginger to the tea bags available in their offices.

Image copyright Zach Marks and Resham Gellatly

A popular ingredient in north Indian chai, ginger is believed to have numerous health benefits and is thought to keep your body warm in winter. The spicy root has been used in hot, milk-based beverages in India for hundreds of years, so when the British popularised tea in the late 19th and early 20th Centuries, adding ginger to the mix was a natural thing to do.

Image copyright Zach Marks and Resham Gellatly

Shobhan Barwa's stand is in the heart of Alipore, a posh neighbourhood in Calcutta. During the annual Hindu festival of Durga Puja, crowds flock here to see the elaborate pandals - temporary structures housing Hindu deities. He usually closes shop by 22:00, but for the week of Durga Puja, he stays open until 05:00 serving chai, eggs and French toast to visitors who need a caffeine kick to keep them going through the night.

Image copyright Resham Gellatly

On the final day of the festival, thousands join a procession to the Hooghly River where giant statues of Hindu gods and goddesses are immersed in the water. Last year, though, celebration turned to tragedy - a young man drowned when a statue fell on him at Baghbazaar. People gathered at a nearby chai stand to mourn his death.

Image copyright Zach Marks and Resham Gellatly

Several decades ago, chai was served in small clay pots, known as kulhar in Hindi or bhar in Bengali. While the bowls are still popular in Calcutta, plastic cups, tiny glasses and steel tumblers have become the vessels of choice across most of India.

Image copyright Zach Marks and Resham Gellatly

Born on a tea estate in Kotagiri in the southern state of Tamil Nadu, Rukmani has been plucking tea leaves all her life. As the eldest member of her group of female workers, she is called Amma, or mother. At lunchtime Amma prepares black chai over a fire of twigs with tea dust provided by a nearby factory.

Image copyright Zach Marks and Resham Gellatly

Ganesh, a chai wallah at the railway station in Patna in Bihar state, brews one of his last pots of the night. Many Indians associate rail travel with the cries of "chai, chai," from tea sellers carrying kettles along trains and platforms. Ganesh has memorised the local timetable and often gives travellers directions as well as a cup of chai.

Image copyright Zach Marks and Resham Gellatly

Chai stands are often family businesses spanning many generations. The owner of this tea stall in Varanasi, one of India's oldest and holiest cities, took over the business when his father passed away and keeps his memory alive by hanging a fresh garland of marigolds every morning.

Image copyright Zach Marks and Resham Gellatly

Lalu Yadav has seen hundreds of thousands of cremations - his chai stand is next to Manikarnika Ghat, a cremation ground by the sacred Ganges river in Varanasi. Many Hindus believe it is an auspicious place to die and the people who come here to wait out their last days ensure the fires of Manikarnika burn bright day and night. Lalu's father started the tea stand 40 years ago, offering mourning families a place to sit and sip chai during cremations. For him, constant proximity to death has become part of life. "There is no sadness here. We are used to seeing this 24 hours a day. These are only bodies."

Photographs by Resham Gellatly and Zach Marks - you can see more pictures and read their blog at chaiwallahsofindia.com.