You know, now all the other part of the stem definitely seems to be completely straight.
And in the whole your tree looks not bad at all, but in details it is far from natural state. Too few branches, wrong shapes, let alone leaves. Shades lack almost at all, the shade on the stem is wrong (at least not natural), too...
I am really sorry that I can`t show you instead of just saying about mistakes... May be one day I will draw a tree. In the summer, perhaps..
Well. My advice is to watch real trees, then watch them again, then draw, then watch, then draw, then watch again and again... The trick is that all realistic genres of art in fact are just copying of nature. Because everything what does not match the nature`s shapes looks weird and, consequently, less people like it. So a real artist is a plagiator. If to say roughly. And to make an art looking naturally means to know the nature quite well... I think you understand the further logical chain.