I recently came across a story of Lord Vishnu, where he hidden himself from gods in root of the plants. So my questions are:
What is the story of Lord Vishnu hiding himself in root of plants, why he did so and where this story is mentioned?
The story of Lord Vishnu Hiding himself in roots of the plants is mentioned in Kanda one- adhyaya two - brahmana five of Satapatha Brahmana Verse 1-11. This story is related to dwarf avatara of Vishnu called as "Vamana" and Vishnu as sacrifice i.e. Yajna and tells us about how gods with the help of Vishnu gained the whole of the earth from themselves from asuras ,who wanted it all for themselves.
The Gods and Asuras both were children of Prajapati , they sprung from him. Once the gods became weak and asuras thought that this world belongs to them only for sure. Then the asuras decided to divide the earth between them.
Now after knowing that the asuras are dividing the earth , the gods thought that this is there share too and asked the asuras for their share. The Asuras agreed and said that "We will give you as much earth as this Vishnu's size i.e. But Lord Vishnu as vamana was dwarf at that time.As Vishnu was dwarf the asuras though that this will be a share of very small size , the size of Vamana.
But the gods were not offended by this as they were knowing that Vishnu is sacrifice (Yajna) and they will get the land equal to sacrifice i.e. how big the sacrifice could be. Now the gods started laying down Vamana on the ground. They laid him eastwards and enclosed him from three sides with chandas (meters). They kept agni on top of him and worshipped Lord Vishnu.
But Vishnu as sacrifice is not limited but is limitless , so instead of using Vamana avatara which was dwarf gods used Vishnu's yajna purusha avatara as measure ,and thus they obtained whole of the earth for themselves as sacrifice (yajnas) can be very great in size also.
Lord Vishnu then got tired as gods enclosed him from three sides by meters. He escaped and then hide in the root of the plants. The gods were surprised as they enclosed hin there was no scope of escaping. Then they dig the ground slightly i.e. 3 inches deep and discovered him in the root of the plants.
This story is actually the part of procedure to be performed during a yajna and tells us the reason of that part. The alter of Yajna is called as Vedi just because Vishnu was discovered below the ground and that marks the depth of alter or yajna vedi i.e 3 inches.
Then second time gods enclosed Vishnu in soil and praised him as "you are good soil and the auspicious" thus they obtained whole of the earth and made it pleasant and soft and fertile and auspicious and they made it abounding in food and drink.
The gods and the Asuras, both of them sprung from Prajāpati, were contending for superiority. Then the gods were worsted, and the Asuras thought: 'To us alone assuredly belongs this world!
They thereupon said: 'Well then, let us divide this world between us; and having divided it, let
us subsist thereon!' They accordingly set about dividing it with ox-hides from west to east.
The gods then heard of this, and said: 'The Asuras are actually dividing this earth: come, let us go to where the Asuras are dividing it. For what would become of us, if we were to get no share in it?' Placing Viṣṇu, (in the shape of) this very sacrifice, at their head, they went (to the Asuras).
They then said: 'Let us share in this earth along with yourselves! Let a part of it be ours!' The Asuras replied rather grudgingly: 'As much as this Viṣṇu lies upon, and no more, we give you!'
Now Viṣṇu was a dwarf1. The gods, however, were not offended at this, but said: 'Much indeed they gave us, who gave us what is equal in size to the sacrifice.'
Having then laid him down eastwards, they enclosed him on all (three) sides with the metres, saying (Vāj. S. I, 27), on the south side, 'With the Gāyatrī metre I enclose thee!' on the west side: 'With the Triṣṭubh metre I enclose thee!' on the north side: 'With the Jagatī metre I enclose thee!'
Having thus enclosed him on all (three) sides, and having placed Agni (the fire) on the east side, they went on worshipping and toiling with it (or him, i.e. Viṣṇu, the sacrifice). By it they obtained (sam-vid) this entire earth; and because they obtained by it this entire (earth), therefore it (the sacrificial ground) is called vedi (the altar). For this reason they say, 'As great as the altar is, so great is the earth;' for by it (the altar) they obtained this entire (earth). And, verily, he who so understands this, wrests likewise this entire (earth) from his rivals, excludes his rivals from sharing in it.
Thereupon this Viṣṇu became tired; but being enclosed on all (three) sides by the metres, with the fire on the east, there was no (means of) escaping: he then hid himself among the roots of plants.
The gods said: 'What has become of Viṣṇu? What has become of the sacrifice?' They said: 'On all (three) sides he is enclosed by the metres, with Agni to the east, there is no (way of) escaping: search for him in this very place!' By slightly digging they accordingly searched for him. They discovered him at a depth of three inches (or thumb's breadths): therefore the altar should be three inches deep; and therefore also Pāñci made the altar for the Soma-sacrifice three inches deep.
This, however, one must not do. Among the roots of the plants he (Viṣṇu) hid himself: therefore let him (the Adhvaryu) bid (the Āgnīdhra) to cut out the roots of the plants. And since they found (anu-vid) Viṣṇu in that place, therefore it is called vedi (altar).
When they had found him, they enclosed him with a second enclosure, saying (Vāj. S. I, 27), 'Of good soil art thou, and auspicious art thou!' on the south side; for when they had thus obtained this earth they made it of good soil and auspicious;--'Pleasant art thou, and soft to sit upon!' they said on the west side, for when they had thus obtained this earth, they made it pleasant and soft to sit upon;--'Abounding in food and drink art thou!' they said on the north side, for when they had thus obtained this earth, they made it abounding in food and drink.
This is story telling us that how gods obtained whole earth and made it rich , soft with which we can grow our food and obtain our drinks and made it habitable with the help of Lord Vishnu.