Leaf Form & Arrangement

Leaf Form & Size, Leaflet Arrangement, & the Petiole

by Bob Harms email-here

Leaf Form

B. trifoliolata leaf forms.
(For larger outline images click here. For original leaf images click on the caption below the leaves.)



Typical sun leaves.	Less common multitoothed leaf.	Shade leaf.

Typical B. swaseyi and hybrid leaf forms.
(For larger outline images click here. For original leaf images click on the caption below the leaves.)



B. swaseyi	Hybrid (H8)

Leaf Length & Width

Leaf length (including petiole) and width measurements are given below. Sun and shade differences are clear. Perhaps the best indicator of species differences is the average length/width ratio, where averaging has smoothed out the overlapping range extremes.
— For both sun and shade, B. swaseyi > Hybrid > B. trifoliolata.

Leaf length (including petiole) & width ranges (in cm.):
	B. swaseyi		B. trifoliolata		Hybrids
	Length	Width	Length	Width	Length	Width
sun	5.8–7.2	2.6–4.3	2.6–7.2	3.4–9	5.4–6.8	3.4–4.5
shade	7.6–14.7	3.3–3.9	4.5–14.7	6.3–8	6.3–14.5	4–6.7

Average leaf length/width ratios:
	B. swaseyi	B. trifoliolata	Hybrid
sun	1.9	0.8	1.5
shade	3.1	1.0	2.0

Leaflet Arrangement on the Rachis


B. trifoliolata	B. swaseyi
(Leaflet main vein in yellow, petiole–rachis in black.)

As the following table shows the lateral leaflets of B. trifoliolata commonly approximate a right angle to the base of the terminal leaflet, but are sometimes more acutely positioned. In contrast both B. swaseyi and the hybrids have leaflets directed at a more acute angle toward the leaf's distal end.

Leaflet Pair No.	I	II	III	IV
B. trifoliolata
Average	83.9°	–
Range	62° – 104°	–
Hybrid
Average	68°	67.2°	–
Range	51° – 85°	50° – 86°	–	–
B. swaseyi
Average	61.9°	66.7°	66.4°	60.5°
Range	45° – 80°	50° – 88°	45° – 85°	47° – 74°

Avoidance of Leaflet Overlap

The one common principle of leaflet arrangement and form for both species is:

Leaflet overlapping is to be avoided.


B. repens leaf.	B. swaseyi distal leaflets	B. trifoliolata leaflet base

Although virtually without exception among species with compound leaves, the application of this principle seems to be somewhat more complex among our species. Given that leaflets are sessile at the rachis nodes, at an acute angle, and more or less truncate, leaflet dispersal along the rachis as well as leaflet to leaflet contact at each node can be shown to interact to minimize overlap. For example, the topmost truncated lateral leaflets of B. swaseyi, are separated from the terminal leaflet base (which is also commonly truncate) by means of a rachis 'stalk' (similar to B. repens; unlike B. trifoliolata). Again, the truncation of B. trifoliolata leaflets is narrow and acute, thus providing space for all three at the node.

But certain hybrids with select features from both species actually increase leaflet overlap. For details and illustrations of overlap at the leaflet base.

Leaflet Internodes

B. trifoliolata lateral leaflets attach directly to the petiole, and hybrids most commonly have only two leaflet pairs, the topmost attached to the same rachis node as the terminal leaflet and the other at a distance below it that would prevent leaflet overlap. The multileafleted B. swaseyi illustrates Ahrendt's (1961, p.8) observation:

... the internodes between the leaflets almost invariably decrease in length ... in passing from the base of the rachis to the apex. ... This feature of decreasing leaflet internodes is less marked in the American Mahonias, because the fewer pairs of leaflets ... scarcely provide a sufficient range for the character to become apparent.
[Note: Ahrendt (pp. 8-9) was also aware of the special short petiole of certain Berberis species. Our B. swaseyi follows that pattern.]

B. swaseyi leaflet internode intervals (in mm.) along the rachis are shown in the following table, for leaves with 3—4 leaflet pairs.

	Petiole	n-3—n-2	n-2—n-1	n-1—n	n—terminal
Average	2.2	15.7	13.5	12.1	6.2
Range	.5—5	4.5—39	8.1—23.5	8.7—17.5	0—15

Variation in B. swaseyi interval length varies according to the following general patterns:

The petiole is always shorter (and generally significantly so) than the next interval.
All n to term intervals are shorter than the previous interval; and in a few instances unstalked.
For others a given interval is shorter than the previous interval for 72% (or 80% if equal or shorter) — of 68 intervals measured.

Petiole Length

Persistent petioles.

B. swaseyi	Hybrid

The most striking difference in petiole length is B. swaseyi's short (0.5 - 5 mm.) distance between the leaf base and its reduced basal leaflet pair. Individual hybrid leaves also have shorter petioles, but on average — even on a bush with short-petioled leaves — their length approaches that of B. trifoliolata.

Petiole length ranges & averages (in mm.):
	Range	Average
B. trifoliolata	6.8-52	20.6
B. swaseyi	0.5 - 5.5	2.2
Hybrid	(2.5) 6.5 - 29 (34)	17