Theory:

     The thought of a world without trees became a sort of nightmare to me and I helped Grandfather in his tree-planting with greater enthusiasm. And while we went about our work, he taught me a poem by George Morris:
 
Woodman, spare that tree!
Touch not a single bough!
In youth it sheltered me,
And I’ll protect it now.
 
     ‘One day the trees will move again,’ said Grandfather. 'They’ve been standing still for thousands of years but there was a time when they could walk about like people. Then along came an interfering busybody who cast a spell over them, rooting them to one place. But they’re always trying to move. See how they reach out with their arms! And some of them, like the banyan tree with its travelling aerial roots, manage to get quite far.’
 
     We found an island, a small rocky island in a dry river-bed. It was one of those river-beds so common in the foothills, which are completely dry in summer but flooded during the monsoon rains. A small mango tree was growing on the island. ‘If a small tree can grow here.’ said Grandfather, ‘so can others.’ As soon as the rains set in and while rivers could still be crossed, we set out with a number of tamarind, laburnum, and coral tree saplings and cuttings and spent the day planting them on the island.
Explanation:
 
As grandfather explained to the author, the thought of a world without trees became like a scary dream to him. So he helped his grandfather with planting trees with more interest and involvement.
 
His grandfather also taught him a poem written by George Morris. The poet tells a woodman to let go of the tree that he is about to cut and that he should not touch a single branch of the tree as it had sheltered him when he was young. It was the poet's turn to protect the tree now.
 
lumberjack-3064802_1920.jpg
Woodman, Don't cut the tree!
  
The grandfather continued his explanation. He said one day the trees would move again. There had been a time when trees walked like people. A troublemaker came along, and he used magical powers to stop them from moving and fixed them deeply in the ground. The trees have been standing motionless for thousands of years because of this spell. But they will always try to move, explained the grandfather. He asked him to see how far they have stretched out their arms. He quotes the example of the banyan tree which has managed to move far with its roots in the air.
 
banyan.jpg
Banyan tree with aerial roots.
  
Later, they found a small island filled with rocks in a dry river-bed area which became very dry during summers but will get submerged in water during monsoon rains. They saw a small mango tree growing on that island. His grandfather said if a small tree can grow in such dry conditions, trees can grow anywhere.
 
flowers in rock.jpg
If plants can grow in dry rocks, it can grow anywhere!
 
As soon as the monsoon set the rains began; the rivers were not flooded because the rain had just started - so one could cross the river easily. They started with many saplings and cuttings of tamarind, laburnum and coral trees and spent the whole day planting them on the island.
 
No.
Words
Meanings
1
nightmarea frightening dream
2
enthusiasm
intense interest, eager enjoyment
3
sparehold back from killing/injuring
4
bougha main branch of a tree
5
interferingstopping, disturbing
6
busybodya meddling or prying person, troublemaker
7
aerialin the air
8
cast a spellto use magic to make something happen to someone
Reference:
State Council of Educational Research and Training (2019). Term-1 English Standard-6. When the Trees Walked - Ruskin Bond (pp. 107-122). Published by the Tamil Nadu Textbook and Educational Services Corporation.