Question

why rheo leaves are boiled in water first and then a drop of surar syrup is put on it?

Answer 1

When rheo leaves will be boiled, it will shrink due to high temperature.
The solute content inside the leaves will come out due to osmosis at first while heating.
Further heating and boiling will destroy the cells.
Cells will be dead.
So, after addition of sugar syrup no more osmosis will take place.