Maeve,
Looking at how stems are like straws is a great example of inquiry. You have offered a suitable prompt that helps the children get interested, and given the analogy of ‘straws to stems’ so that the children have a scientific explanation that they can test.
With an older class, they might have further ideas — what happens if the stem is bent? what happens if you make a little nick in the flower stem? what happens if you split the stem and put half in one colour and half in another?
Steve Spangler has some suggestions around this: https://stevespangler.com/experiments/colorful-carnations/