image processing - Detecting Central Axis (petiole/rachis) of a Pinnated Palm

I'm new in StackExchange and I'd appreciate some help. I'm working in an algorithm for detecting "aguajes", a particular kind of palm. So far, I got good results using a texture feature extractor and other characteristics, but the main problem is that I can't difference between an "aguaje" and other palms (mainly pinnated palms) because the texture is very similar.

1.AGUAJE

2.Pinnated Palm

I see that the main difference in the second one is the presence of a Central Axis (also called Petiole or Rachis). The question is "How to detect it?". I mean, if I'm able to detect the presence of a Petiole, I'd be able to distinguish between this two types of palm. I think color is not an important issue because even when there's no color (gray image), we can discern where is located that Axis.

Thanks in advance.