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 and how it is specifically applied in botany today.
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. This is useful to know when pruning woody plants. 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, then the axis would be its rachis. How flowers are arranged on the rachis and the sequence of anthesis (opening of the flower bud) decide the type of inflorescence. For example, if flowers open from the bottom of the axis to the top or from the outside to the inside, then you know the inflorescence is indeterminate. The opposite is true for determinate inflorescences—flowers on the rachis open from the top down or from the center outward. Why do these terms matter to a gardener?
Left to right: Indeterminate ‘Moneymaker’ tomatoes, indeterminate vs determinate growth habit, determinate tomatoes.
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)
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 compound leaf, then the axis would be the rachis (main vein or midrib) that separates the leaflets. Secondary veins are called rachillae. For example, native Carya ovata (shagbark hickory) has a compound leaf that is odd-pinnate—it has an odd number (5–7) of broadly lanceolate leaflets along the rachis with the terminal leaflet being the largest. 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 ovata and Dryopteris marginalis.
You can recognize a leaf because in its axil is 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.
Note: Click on images to see enlarged photos, captions, and photo attributions.
On a mobile phone, click on the information symbol (circle with a letter ℹ︎ symbol).
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.