diff --git a/fern_sim/Cargo.toml b/fern_sim/Cargo.toml
index 73e36a9..cc608ae 100644
--- a/fern_sim/Cargo.toml
+++ b/fern_sim/Cargo.toml
@@ -1,6 +1,7 @@
 [package]
 name = "fern_sim"
 version = "0.1.0"
+edition = "2018"
 authors = ["You <you@example.com>"]
 license = "MIT"
 homepage = "https://fernsim.example.com/"
diff --git a/fern_sim/src/cells.rs b/fern_sim/src/cells.rs
deleted file mode 100644
index 3697d5d..0000000
--- a/fern_sim/src/cells.rs
+++ /dev/null
@@ -1,12 +0,0 @@
-//! The simulation of biological cells, which is as low-level as we go.
-
-pub struct Cell {
-    x: f64,
-    y: f64
-}
-
-impl Cell {
-    pub fn distance_from_origin(&self) -> f64 {
-        f64::hypot(self.x, self.y)
-    }
-}
diff --git a/fern_sim/src/lib.rs b/fern_sim/src/lib.rs
index ad94c18..1c51c66 100644
--- a/fern_sim/src/lib.rs
+++ b/fern_sim/src/lib.rs
@@ -1,7 +1,9 @@
 //! Simulate the growth of ferns, from the level of
 //! individual cells on up.
 
-pub mod cells;
+#![warn(rust_2018_idioms)]
+#![allow(elided_lifetimes_in_paths)]
+
 pub mod plant_structures;
 pub mod simulation;
 pub mod spores;
diff --git a/fern_sim/src/plant_structures/leaves.rs b/fern_sim/src/plant_structures/leaves.rs
index 50d63f5..dce330a 100644
--- a/fern_sim/src/plant_structures/leaves.rs
+++ b/fern_sim/src/plant_structures/leaves.rs
@@ -2,5 +2,5 @@
 //! Simulation of individual leaves (for the formation of leaves, see `stem`).
 
 pub struct Leaf {
-    x: bool
+    pub x: bool
 }
diff --git a/fern_sim/src/plant_structures/mod.rs b/fern_sim/src/plant_structures/mod.rs
index 32ba8eb..723cc9b 100644
--- a/fern_sim/src/plant_structures/mod.rs
+++ b/fern_sim/src/plant_structures/mod.rs
@@ -43,3 +43,17 @@ impl Fern {
         self.stems.iter().all(|s| !s.furled)
     }
 }
+
+/// Create and return a [`VascularPath`] which represents the path of
+/// nutrients from the given [`Root`][r] to the given [`Leaf`](leaves::Leaf).
+///
+/// [r]: roots::Root
+pub fn trace_path(leaf: &leaves::Leaf, root: &roots::Root) -> VascularPath {
+    VascularPath { from: leaf.x, to: root.x }
+}
+
+#[doc(alias = "route")]
+pub struct VascularPath {
+    pub from: bool,
+    pub to: bool,
+}
diff --git a/fern_sim/src/plant_structures/roots.rs b/fern_sim/src/plant_structures/roots.rs
index d40b4f3..148f7b1 100644
--- a/fern_sim/src/plant_structures/roots.rs
+++ b/fern_sim/src/plant_structures/roots.rs
@@ -1,7 +1,7 @@
 #![allow(dead_code)]
 
 pub struct Root {
-    x: bool
+    pub x: bool
 }
 
 pub type RootSet = Vec<Root>;
diff --git a/fern_sim/src/plant_structures/stems.rs b/fern_sim/src/plant_structures/stems.rs
index 70a8123..c85e81c 100644
--- a/fern_sim/src/plant_structures/stems.rs
+++ b/fern_sim/src/plant_structures/stems.rs
@@ -3,6 +3,10 @@
 //! the feathery leaf structure that's the most immediately recognizable
 //! property of ferns.
 
+// in plant_structures/stems.rs
+pub mod xylem;
+pub mod phloem;
+
 pub struct Stem {
     pub furled: bool
 }
diff --git a/fern_sim/src/plant_structures/stems/phloem.rs b/fern_sim/src/plant_structures/stems/phloem.rs
new file mode 100644
index 0000000..0ee6037
--- /dev/null
+++ b/fern_sim/src/plant_structures/stems/phloem.rs
@@ -0,0 +1,6 @@
+//! Structures for distributing the products of photosynthesis.
+
+/// Tissue for translocating sucrose and other photosynthesis products.
+pub struct Phloem {
+    pub flow_rate: f32,
+}
diff --git a/fern_sim/src/plant_structures/stems/xylem.rs b/fern_sim/src/plant_structures/stems/xylem.rs
new file mode 100644
index 0000000..3278326
--- /dev/null
+++ b/fern_sim/src/plant_structures/stems/xylem.rs
@@ -0,0 +1,6 @@
+//! Structures for bringing nutrients from roots up to other parts of the plant.
+
+/// Vascular tissue for transporting water and nutrients from the roots.
+pub struct Xylem {
+    pub flow_rate: f32,
+}
diff --git a/fern_sim/src/simulation.rs b/fern_sim/src/simulation.rs
index 9fb16b6..9c0c5f8 100644
--- a/fern_sim/src/simulation.rs
+++ b/fern_sim/src/simulation.rs
@@ -4,7 +4,7 @@
 
 use std::fs::File;
 use std::time::Duration;
-use plant_structures::{Fern, FernType};
+use crate::plant_structures::{Fern, FernType};
 
 /// The simulated universe.
 pub struct Terrarium {
diff --git a/fern_sim/src/spores.rs b/fern_sim/src/spores.rs
index a6cfd91..5c51851 100644
--- a/fern_sim/src/spores.rs
+++ b/fern_sim/src/spores.rs
@@ -2,27 +2,46 @@
 
 //! Fern reproduction.
 
-use cells::Cell;
+use cells::{Cell, Gene};
 
 /// A cell made by an adult fern. It disperses on the wind as part of
 /// the fern life cycle. A spore grows into a prothallus -- a whole
-/// separate organism, up to 5mm across -- which produces a zygote,
-/// which becomes a new fern. (Plant sex is complicated.)
+/// separate organism, up to 5mm across -- which produces the zygote
+/// that grows into a new fern. (Plant sex is complicated.)
 pub struct Spore {
-    x: bool
-}
-
-/// A compartment, usually on the bottom of a leaf, where spores form.
-pub struct Sporangium {
-    x: bool
+    size: f64
 }
 
 /// Simulate the production of a spore by meiosis.
 pub fn produce_spore(factory: &mut Sporangium) -> Spore {
-    Spore { x: false }
+    Spore { size: 1.0 }
+}
+
+/// Extract the genes in a particular spore.
+pub(crate) fn genes(spore: &Spore) -> Vec<Gene> {
+    todo!()
 }
 
 /// Mix genes to prepare for meiosis (part of interphase).
 fn recombine(parent: &mut Cell) {
+    todo!()
 }
 
+pub struct Sporangium;
+
+mod cells {
+    //! The simulation of biological cells, which is as low-level as we go.
+
+    pub struct Cell {
+        x: f64,
+        y: f64
+    }
+
+    impl Cell {
+        pub fn distance_from_origin(&self) -> f64 {
+            f64::hypot(self.x, self.y)
+        }
+    }
+
+    pub struct Gene;
+}
diff --git a/fern_sim/tests/unfurl.rs b/fern_sim/tests/unfurl.rs
index c60f078..59adc10 100644
--- a/fern_sim/tests/unfurl.rs
+++ b/fern_sim/tests/unfurl.rs
@@ -1,6 +1,8 @@
 // tests/unfurl.rs - Fiddleheads unfurl in sunlight
 
-extern crate fern_sim;
+#![warn(rust_2018_idioms)]
+#![allow(elided_lifetimes_in_paths)]
+
 use fern_sim::Terrarium;
 use std::time::Duration;