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One month ago I bought a bonsai. The only information I get that it needs >18°C and light. Well, I put it to the window sill, and it is still there. In the morning, it gets direct sunlight for hours. A few (4-5) days after I bought, I recognized that its leaves starts to wither. It continued, the leaves dried and fall off. I used to water it with simple bottle, but now I water it with spray, both the soil and the leaves (not too much water, but I try to keep the soil wet). Some of its roots are not completely in the soil. The interesting fact for me, is that a contiguous part of its leaves are healthy (so far). (See the pictures below).
What is the source of the problem? Why is my tree falls off its leaves? What should I do to revive my bonsai?
As I searched on the internet, i think it is a Serissa Foetida Variegata.
I hope it is not too late and You can help me before my tree loses all of its leaves. Thank You.

Marcellcx wrote: One month ago I bought a bonsai.
A few (4-5) days after I bought, I recognized that its leaves starts to wither. It continued, the leaves dried and fall off.

Probably a combination of incorrect watering, the wrong location and a low quality plant. Serissas are not easy. Seems it is dying from the bottom (lowest branches to go first). Probably the roots are dead already.

Recovery is unlikely.