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Joan entered the kitchen of the brownstone early one Friday evening for a much-needed break while solving a case, to find Ms. Hudson busy at their table.
“Ms Hudson, hi! What are you doing?” Joan asked as she poured a cup of coffee.
Ms. Hudson looked up from the spread of several dozen cut white tulips fully covering the tabletop. “Hello, Joan! I’m dyeing these tulips for my friend’s wedding tomorrow.” She waved at the blossoms. “He asked me to provide floral centrepieces for the dinner. I’m making them here because my kitchen’s too small.”
“How nice, my congratulations to the couple.” Joan then noted clear glass vases full of deep blue liquid. “How does that work?”
Ms. Hudson took a knife and deftly cut a diagonal on the stems of several tightly closed tulips. “Blue tulips don’t exist in nature,” she began, “but you can colour them blue by putting white flowers in food colouring.”
She checked one test tulip in a clear glass vase full of dye. “The dye travels up the xylem in the stem to the flowers, which turns the flowers blue. Look, you can watch it in progress.”
Joan peered at the blue colour climbing slowly, but visibly up the stem and through the petals. “The xylem delivers water up from the roots, right? If I’m remembering my botany classes correctly.”
“Yes, and the xylem transports nutrients too, all through the plant,” Ms. Hudson agreed. “You can see this tulip is almost done.”
Joan observed the thread of deep blue dye that had earlier crept along the veins of the petals, until the entire flower was a brilliant cerulean. “I suppose you can control how dark the flower ends up?”
“Oh yes,” Ms. Hudson said, “my test took about an hour to reach that colour.”
She withdrew the flower from the colouring and placed the cut stem end into a small vial filled with flower food. She then cut the stems of the rest of the tulips.
“These will stay in a bit longer. I’ll check them later. They will go in with the carnations tonight, and when the dye migrates naturally into the water the carnations will develop a blue tint as they take it up. They make very stunning arrangements.” She piled the tulips into several vases containing blue food colouring.
Joan took a sip of coffee. “So not just tulips.”
“This works with any plant. Carnations, roses, lilies—”
Joan blinked. “Plants in soil too?”
“I suppose you can dump the dye in the soil if you wanted, though it may be not too efficient. However, whatever can get absorbed into the root will be transported via the xylem.”
“If the xylem can carry molecules as large as food colouring…” She inhaled sharply. “That’s it. That has to be it.”
“I beg your pardon?”
“Ms. Hudson, I think you just solved our case,” Joan said, growing excited. “Thank you.” She reached out and squeezed her arm.
“Oh, you’re very welcome. I’m always happy to help.” She returned to her tulips.
Joan strode out of the kitchen, calling, “Sherlock? I think I know how our murder victim was exposed to the poison now.”