I made a tree with Add curve -> Add tree, then I added a plane with a particle system of type hair; under the Render panel I selected Object and chose the tree above mentioned as the Dupli Object.
The problem is that Blender rotates the trees making them lie on the plane.
I encountered the same problem with other mesh objects as Dupli objects.
Could you tell me where am I doing wrong, please?
Thanks in advance.
In EDIT MODE, rotate the tree 90 degrees around the X axis (R X 90)
If you want the original tree to be upright again, rotate it back in OBJECT MODE
I have done as you suggested, but in the case of a bird wing it does not work: after combing the hair particles, some feathers are rotated in one direction and some others into the perpendicular one.
I don’t know where feathers came from in your question but as for trees see the demo: http://screencast.com/t/dhac69IO
In my question I mentioned only trees because I had that problem and I thought that the problem of the feathers was of the same nature.
To explain better the trouble I am in, I send you a screen shot.
Thanks in advance
looks like you need to either model low poly feathers and texture them or add a transparency map. you also sometimes have to scale the parent object in edit mode. Note, you can move it to a new layer and still have it work 
The feathers are low-poly, but what I am trying to do is to keep the feathers parallel to the XY plane; as you can see in the screen shot the plane of the middle feather is perpendicular to the XY plane.
Let me know if it is possible to do what I was trying to do.
In the meantime I will follow David Ward’s method taught on BlenderCookie.
Thanks for your attention.
hmm, I dont know how to say it but this video tut can tell you how to paint low poly things with blender’s hair system http://cgcookie.com/blender/2009/02/18/painting-instanced-objects/
I watched the tutorial you suggested. To correct the rotation of the feathers I pressed Normal as the initial rotation axis and now they are rotated as I wanted.
There is only one thing that does not work: in particle mode I combed the hair but in object mode the feathers don’t follow the direction of the hair; the other particle mode tools (add, length) work well
that is because once you use the hair to make objects then it is no longer hair. It will only scale the object. In order to have different directions of feathers I suggest that you make more than one feather and when you want them to turn disconnect the “hair” then call the next feathers. He explains this if you listen carefully. It took me a few times of going over it to understand that.
thank you very much
Believe it or not I havent actually tried this but I have done so much research on hair that I know the basics of making hair and hair based systems. I plan on using this very same system to make grass and distance trees and stuff like that. I really need to start applying it to my own models but I’ve been busy working on the rigging and modeling of the head. I just finally decided on human like eyes now I need to decide on other things about said model, but first I really need to clothe the poor woman, she’s been running around in her birthday suit and getting all embarrassed on me. 