by Bob Harms 

Features Common to Both Terminal and Lateral Leaflets

• Apical lobes and marginal teeth are acute (if determined by lamina margins, excluding terminal spines), often minimally convexly rounded, and spinose.
  
  Typical leaf tips. (Black lines for lamina; red, for lamina & spine.)
  
• Commonly asymmetrically oblique, sometimes resulting in subfalcate forms.
  
  B. swaseyi leaf asymmetry (left). — B. trifoliolata leaflet base.
  
• Spine-tipped teeth are more or less evenly spaced along the margin.


• Attachment is sessile at a rachis node (hybrid terminals very rarely with a petiolule).


• Leaflet bases are normally truncate, narrowly so with B. trifoliolata (cf. above image). With B. swaseyi leaflets are often abruptly truncated at the lowest tooth, which may be significantly reduced. With B. swaseyi and hybrids, terminal leaflets are occasionally acute and not truncate. One B. swaseyi bush was noted with predominantly acute terminal bases.
  
  B. swaseyi reduced proximal teeth (left). — Hybrid nontruncate (acute) terminal base.

Features That Vary by Position and Species

Range of terminal leaflet types - at the same scale (mm., shown).

With palmately trifoliate B. trifoliolata there is no significant difference between terminal and lateral leaflets, which are oblong to lanceolate (to linear–lancolate); length/width ratio 2.2 – 8.1, average c. 4.5. The base is narrowly acute–truncate.

B. swaseyi has numerous leaflet differences by position, as shown in the tables below. Terminal leaflets are elliptic to lanceolate; length/width ratio 1.7 – 3.9, average c. 2.7. Lateral leaflets are shorter but tend to be wider, giving overall length/width ratios 1.0 – 2.7 (see the table below for averages).

Leaflets of individual hybrids tend to vary between forms typical of the two species, as indicated below.

Terminal leaflets are generally longer than the next adjacent lateral leaflets, although not significantly so with B. trifoliolata. B. swaseyi showed a clear correlation of this relationship with sun/shade exposure, lateral leaflets in full sun being significantly shorter. Hybrids regularly have shorter lateral leaflets.

Length of topmost lateral leaflet as percentage of terminal leaflet length.

With B. swaseyi (apart from the greatly reduced basal leaflets) leaflet pairs below the topmost pair tend to reduce gradually in both size and the number of teeth on the margins. Hybrid lateral leaflets are much less reduced and closer to those of B. trifoliolata in this respect.

Marginal teeth (spines) per leaflet.

	Terminal leaflet	First lateral leaflet	Next lateral leaflet	Basal reduced leaflet	Tooth height mm.
B. trifoliolata	1 – 3	1 – 3	—	—	2.1 – 10
	avg. 1.7	avg. 1.5	—	—	avg. 4.5
B. swaseyi	(3–) 4 – 8 (–9)	(2–) 3 – 6 (–7)	(2–) 3 – 5 (–6)	(0–) 1 – 3 (–5)	0.1 – 2.5
	avg. 5.9	avg. 4.1	avg. 3.8	avg. 2.4
Hybrids	2 – 5	2 – 3 (–4)	(1–) 2 – 3	—	0.1 – 3.0
	avg. 2.9	avg. 2.1	avg. 2.1	—

13-leaflet B. swaseyi shade leaf. (Colored and outlined to show its form.)

B. swaseyi leaflet measurements are given in the followng table. The petiole extends from the stem to the first rachis node for the reduced basal leaflets. After that the distal leaflet pair is given as 'n' and other pairs are enumerated as 'n-[1 – 4] (e.g., for 6 pairs as shown in the image above, the sequence from the base would be 'basal—n-4—n-3—n-2—n-1—n—terminal):

The gradual reduction in leaflet length is not matched by a reduction in width, giving significanly lower length/width ratios - a progression from lanceolate to elliptic toward a suborbicular form.