-( Story related, cannot be missed )-
*While this is "flagged" as possibly story related, the real note is that... you could scrape by without having all 101 Power Cells early on. It would be in your best interest to avoid back-tracking at the end to get this for the final ending. I would advise doing this as soon as possible, as the Blue Eco vents help out, so the sooner the better, as Blue Eco aids in the collection process.
Level: Forbidden Jungle
When you venture into the tower of the Forbidden Jungle, you will eventually find that you have turned on the Blue Eco vents all over the world. This is a GREAT thing, as now you can snag Precursor Orbs and other little tidbits (Scout Flies, Green Eco balls) more easily. The only problem that you will face now is that you have to find a way out. Using the Blue Eco vent near the switch you turn on, run through all of the Precursor Doors and then continue through the tunnel provided.
The Dark Eco Plant is a basic boss. He will have little aphids pop out and stalk you. Attack them when they recede their spikes. All while you are doing this, the plant will try to snap at you, so keep moving and keep your distance. Shortly after you deal with all of the aphids and it has snapped, it will pull its head up and sprout leaves for you to climb up. This is your chance to deal damage to it. Hop up and give it a good whack. Rinse and repeat for two more hits and the plant will wilt, giving up the Power Cell from its gullet and opening the Blue Eco Vent beside it.
*NOTE: Before you use the jump pad, stomp on the Dark Eco's Plant several times to pull out all of the Precursor Orbs from its belly. It will be more of a hassle going through all of that just to get these later.