abaxial [ ab-AK-see-uhl ] adjective: directed away from the axis; (of a leaf) the lower surface or underside
adaxial [ ad-AK-see-uhl ] adjective: on the same side as or directed toward the axis; (of a leaf) the upper surface
axis [ AK-sis ] noun, plural axes: the central part of a longitudinal support on which organs or parts are arranged; the central line of any body
rachis [ REY-kis ] noun, plural rachises, rachides: the axis of an inflorescence or compound leaf or frond
stem [ stem ] noun: the axis of a plant bearing nodes, leaves, and buds
Most of us are familiar with the meaning of axis—the imaginary straight line around which the earth or other bodies rotate—as it was first used in the 1540s by Copernicus. Some of us may be less familiar with a second general definition—the straight line around which parts are arranged—that came into usage in the 1660s, its botanical application today, and how it can be relevant to your gardening activities.
Stem
Simply, if you drew a straight line through the center of a plant, then the axis would be its stem. The stem (or trunk) bears the nodes out of which lateral buds and leaves grow and at its base grow the roots. The central line through each branch is also an axis.
Why is this useful to know?
Consideration of the axis of a stem/branch is essential when you have to remove a diseased limb or shape or prune back a woody plant. A proper cut is usually perpendicular to the axis of the branch being removed. As a rule, this ensures that the branch collar (the bulge at the base of a branch where it attaches to the trunk), which contains the chemicals to help seal the wound and prevent decay, is not injured.
Rachis of an Inflorescence
If you drew a straight line through the center of an inflorescence, the main axis usually would be the rachis. An exception is a capitula, where the main axis is the receptacle. The peduncle supports the entire flower cluster and, when a rachis is present, lies below it. How flowers are arranged on the main axis and the sequence of anthesis (opening of the flower bud) determine the type of inflorescence. If flowers open from the bottom of the axis upward or from the outside inward, then you know the inflorescence is indeterminate—that is, it can continue to grow. The opposite is true for determinate inflorescences—the axis terminates in a flower and the flowers open from the top downward or from the center outward.
Left to right: Indeterminate ‘Moneymaker’ tomatoes, indeterminate vs determinate growth habit, determinate tomatoes.
Why do these terms matter to a gardener?
If you are choosing plants for a kitchen garden, knowing whether, for example, your tomatoes, cucumbers, and beans are indeterminate or determinate, will tell you not only how the fruit will mature but what kind of maintenance the plant might require. If you desire to eat fresh produce over a longer period, then consider indeterminate varieties, whose growth and fruit production are limited only by adverse conditions (such as the first killing frost or disease). These varieties tend to grow like vines, so they require more attention to pruning and staking and are a good option for vertical gardening. If you intend to can or pickle the harvest, then you will find that the fruits of a determinate variety mature earlier in the season all around the same time and soon thereafter the plant begins to die. The limited growth of determinate varieties usually means little to no pruning because the plants tend to be more compact and bushy, making them good candidates for container or small space gardening. Understanding how growth occurs will help inform how long your plant may flower or fruit and how much time and effort you may have to devote to the plant’s upkeep.
Axes and Rachis of a Leaf
Have you ever wondered why leaves grow into different shapes and sizes? A leaf develops along three axes with its cells multiplying in a sequential, coordinated fashion. First cells elongate along the proximal-distal axis (base to tip) and then flatten along the adaxial-abaxial axis (dorsal or upper surface to ventral or lower surface). In due course, cells widen along the medial-lateral axis (center to the margins) creating a symmetric leaf. How a leaf’s cells grow along these axes decides the length, size, and shape of its blade. (Machida et al 2015 and Zhao et al. 2022)
Virginia buttonweed.
Photo © Mary Free
Virginia buttonweed.
Photo © Mary Free
A blade that is subdivided into leaflets (or pinnae) is called a compound leaf or frond. If you drew a straight line through the center of a pinnately compound leaf, then the axis would be the rachis (main vein or midrib) that separates the leaflets. Secondary veins are called rachillae. The arrangement of veins in a leaf is called venation. For example, native Carya tomentosa (mockernut hickory) has a compound leaf that is odd-pinnate—it has an odd number (5–9) of broadly ovate-lanceolate leaflets along the rachis. Its leaf is attached to the stem by a petiole (up to 4.75 inches long), while the leaflets attach to the leaf rachis by petiolules (up to .5 inches long for the terminal leaflet). The frond of native Dryopteris marginalis (marginal wood fern) is bi-pinnately compound—divided twice. The pinnae (leaflets) are arranged on the rachis and the pinnules (secondary leaflets) are on the rachillae.
Left to right: Compound leaves of Carya tomentosa and Dryopteris marginalis.
How can you differentiate a leaf from a leaflet (and why it matters)?
In the axil of a leaf, you will likely find a bud, which has the potential for new growth. If you prune the axis above the axillary bud, then the plant is likely to develop a new shoot. Leaflets do not have axillary buds so if you cut them off, nothing new will grow in their place.
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References
Britannica, The Editors of Encyclopaedia. “flower“. Encyclopedia Britannica, 6 Sep. 2022, https://www.britannica.com/science/flower. (accessed January 14, 2023)
Christensen S, Weigel D. 1998. Plant development: The making of a leaf. Current Biology. 8(18); R643-R645. ISSN 0960-9822.
In regards to tomatoes, what is meant by the terms determinate and indeterminate? Horticulture and Home Pest News. Iowa State University Extension and Outreach. (accessed February 2, 2023).
Logan RS, Laursen SB. 2007. Pruning Deciduous Trees. Montana State University Extension.
Machida C, Nakagawa A, Kojima S, Takahashi H, Machida Y. 2015. The complex of ASYMMETRIC LEAVES (AS) proteins plays a central role in antagonistic interactions of genes for leaf polarity specification in Arabidopsis. Wiley Interdiscip Rev Dev Biol. 4(6):655-71. doi: 10.1002/wdev.196. PMID: 26108442; PMCID: PMC4744985.
Stone-Schmidt S. 2015. What’s The Difference Between Determinate and Indeterminate Tomatoes? UCCE Master Gardeners of San Bernardino County. University of California.
Weakley AS, Ludwig JC, Townsend JF. 2012. Flora of Virginia. Botanical Research Institute of Texas.
Zhao Y, Zhang Y, Zhang W, Shi Y, Jiang C, Song X, Tuskan GA, Zeng W, Zhang J, Lu M. 2022. The PagKNAT2/6b-PagBOP1/2a Regulatory Module Controls Leaf Morphogenesis in Populus. International Journal of Molecular Sciences. 23(10): 5581.