I wlil talk about it here to avoid filling the scene with spell talking 
Maybe i'm wrong due to not having a full fleshed level in English, but i've always thought that:
Straight wall =/= Wall in a straight line
To me it's like a wall that its not inclined, so you cannot do shenanigans like domes, tubes or makeshift bridges.
In fact, in Spanish, the spell it's literally translated as "muro vertical" (vertical wall).
Also, i dont see any logic reason to not be able to spawn every bush a little moved from the other in a circle or a curve. It would be a very weird specific indication to add to the spell, since you can do just exactly the same spell but without that limitation (and with +5 extra paces lenght, since by default its also shorter that it could be).
If im wrong, its ok, but i would like to ask to be able to make some changes to my starting spells, since with this one it would already have 35 spell levels (the other spell being the sun light circle one) that will just be used to gain a bonus for "similar spell" in my lab work, since they dont serve the purpose for which I chose them in the first place.