Get reference to a Particle System created in Scene Editor - swift

Question

I've created a particle system in the Scene Editor (not the particle Editor), and it's named "particles" (this is the default name).
Back in the ViewController, I'm attempting to get a reference to this particle system and change some properties of it.
But I can't figure out why this doesn't work:
let particleSystem = SCNParticleSystem(named: "particles", inDirectory: "")
particleSystem?.isAffectedByGravity = true
I know it's possible to set gravity on within the Scene Editor, but I'm simply using this as a test to see if the reference to the Particle System is working. It's not.
What am I missing or doing wrong?
ADDITIONAL EFFORTS:
As per Rickster's suggestion, trying this:
let particleSystems = scene.particleSystems
let myParticleSystem = particleSystems?[0]
myParticleSystem?.isAffectedByGravity = true
print(particleSystems)
This has now this problem:
My thinking (faulty as it is) was that the array's 0 location would have the only particle system I have in this scene.

Answer 1

Here is a picture of the scene I got:
And the code to get a reference to the particle system assigned to the particles SCNNode instance:
In Swift 2.3 (and earlier)
let particlesNode:SCNNode = scene.rootNode.childNodeWithName("particles", recursively: true)!
let particleSystem:SCNParticleSystem = (particlesNode.particleSystems?.first)!
In Swift 3.0
let particlesNode:SCNNode = scene.rootNode.childNode(withName: "particles", recursively: true)!
let particleSystem:SCNParticleSystem = (particlesNode.particleSystems?.first)!
Alternative Array Index Syntax
let particleSystem:SCNParticleSystem = (particlesNode.particleSystems?[0])!

Answer 2

As the docs for that initializer suggest, it's for loading particle systems that are in their own files. When you make a particle system in the scene editor, the particle system isn't its own file — it's part of the scene file you're making.
To find it... well, first you have to load the SCNScene object that file represents. Assuming you've done that already, you'll need one of two ways to access the particle system depending on how you defined it in the editor:
If the particle system is attached to a node — the straightforward way to position it in the scene — you can use the scene's rootNode property to get at the scene contents, then use methods like this one to find the node containing the particle system using the name you assigned it in the editor. (You did give it a name in the editor, right? If not, go do that... or worst case, there's always this method that lets you search for nodes based on any test you can code.)
If not, use the scene's particleSystems property to find all systems that aren't attached to nodes.