Structure and development of cactus flowers

Flower organization and the significance of the inferior ovary

In an open flower, the colourful tepals and numerous stamens are easiest to see. The portion below them is equally important for understanding its structure. In many cacti, the ovary is inferior, lying below the insertion of the other principal floral organs and surrounded by additional tissues. This arrangement forms during development. It does not result from a completed ovary later being pushed into the stem. [Rosas-Reinhold et al. 2021] [González et al. 2023]

The cactus perianth may consist of numerous elements grading from more scale-like outer parts to distinctly petal-like inner parts. The term tepals is therefore often more useful than imposing a sharp boundary between sepals and petals. Inside lie the stamens and a pistil composed of fused carpels. Organ number, size and arrangement vary. [Rosas-Reinhold et al. 2021] [González et al. 2023]

In many species, scales, hairs or spine-bearing areoles occur on the outer surface of the floral axis. Such structures explain why the cactus flower interests researchers studying the boundary between vegetative growth and reproduction. Appearance alone does not establish the origin of every tissue. Development needs to be followed from its earliest stages. [Rosas-Reinhold et al. 2021] [González et al. 2023]

Pericarpel and pericarp

The pericarpel is the tissue surrounding the ovary in the characteristic organization of a cactus flower. Its origin and relationship with the shoot axis and receptacle have been discussed in the literature. The review by Rosas-Reinhold and co-authors shows that describing a flower as a structure combining shoot and floral features is a developmental hypothesis requiring careful comparison, rather than merely a vivid metaphor. [Rosas-Reinhold et al. 2021] [Ramírez-Castro et al. 2024]

The pericarp is the portion of the fruit developing from the ovary wall. Pericarpel and pericarp are not synonyms. In a mature fruit, tissues of different origins may form a compact structure, making their boundaries difficult to identify. Colour or texture at maturity alone does not determine their origin. [Rosas-Reinhold et al. 2021] [Ramírez-Castro et al. 2024]

The distinction also matters when interpreting sections. A green, fleshy layer outside the ovary need not be its wall. Conversely, the absence of a sharp anatomical boundary does not remove the difference in tissue origin. A section of a mature flower should therefore be compared with a series of earlier stages. [Rosas-Reinhold et al. 2021] [Ramírez-Castro et al. 2024]

Material and methods in the Disocactus study

Ramírez-Castro and co-authors compared development in Disocactus speciosus and D. eichlamii. The first produces more widely open flowers with numerous stamens. The second has more tubular flowers with far fewer stamens. The question concerned both the differences in the completed flower and the developmental stages at which they begin to arise. [Ramírez-Castro et al. 2024]

Active areoles and buds of successive sizes were examined using histological sections and scanning electron microscopy. The authors distinguished nine significant stages, grouped into four broader developmental phases. Sections revealed internal tissue organization, while surface images added information about the shape and distribution of organ primordia. [Ramírez-Castro et al. 2024]

The material came from five D. speciosus plants and one D. eichlamii plant. This is an important limitation. Numerous buds and replicate preparations document successive developmental stages but do not replace broad sampling of independent individuals. The paper provides a detailed picture of the material examined, but does not determine the full variability of both species. [Ramírez-Castro et al. 2024]

Transition from shoot meristem to floral meristem

The observed sequence began with areole activation. A shoot apical meristem appeared, surrounded by spirally arranged leaf primordia. At this stage, the presence of an active meristem alone did not yet allow the entire structure to be treated as a completed flower. The transition to a floral developmental programme occurred later. [Ramírez-Castro et al. 2024]

The floral meristem produced tepal primordia and regions giving rise to stamens and carpels. The perianth developed near previously formed organs with leaf-like features. A continuous axis incorporating vegetative and floral tissues formed. This is a substantially richer picture than a diagram in which a miniature mature flower emerges directly from an areole. [Ramírez-Castro et al. 2024]

The study organizes the process using different specimens collected at successive stages. It reconstructs development from a series of preparations rather than directly following every cell in the same plant. It reveals the sequence of forms and tissue organization, but alone does not establish all the molecular mechanisms of meristem-identity change. [Ramírez-Castro et al. 2024]

Stamen formation and differences between species

In both species, stamens developed from a ring-shaped meristem. In D. speciosus, this region was extensive and remained active for a long time. It produced successive primordia, resulting in about 200 stamens. In D. eichlamii, the ring was smaller and stamen number was about 20. The difference visible in an open flower therefore had a counterpart in the earlier activity of the meristematic tissue. [Ramírez-Castro et al. 2024]

The authors described successive stamen primordia forming centrifugally. This means that later elements appeared farther from the centre of the relevant region. Their direction of initiation should not be confused with later filament elongation or the timing of pollen maturation. [Ramírez-Castro et al. 2024]

The result shows that differences in organ number can be considered through meristem size, duration of activity and organization. It does not demonstrate that the specific genetic change responsible for the difference between these two species has been identified. Establishing such a cause requires other methods, including studies of gene expression and function. [Ramírez-Castro et al. 2024]

Growth around the ovary and floral-tube structure

In successive stages, tissues surrounding the developing ovary grew upwards. At the same time, the region bearing the floral organs grew. Intercalary growth played an important role, occurring within a section of the axis rather than solely at its tip. This development raised the perianth and stamens above the ovary region. [Ramírez-Castro et al. 2024]

Despite a similar general structural plan, tube origin differed between the species examined. In D. eichlamii, the authors described a tube formed with contributions from fused tepal bases and adnate stamen filaments. In D. speciosus, the tube included outer vegetative tissues with leaves and areoles, and inner floral tissues. [Ramírez-Castro et al. 2024]

The term floral tube is therefore useful for describing appearance, but does not itself identify the parts that built it during development. Similarity of shape need not imply full anatomical identity. This is one of the most important findings from comparing these two plants. [Ramírez-Castro et al. 2024]

Pollination, fertilization and seed formation

Pollination is the transfer of pollen to the stigma. It is not yet fertilization. If conditions and biological compatibility allow, a pollen grain germinates and produces a tube. The tube grows through pistil tissues towards the ovules. An ovule is a multicellular structure in which the embryo sac containing the egg cell develops. Equating the ovule with the female gamete overlooks a fundamental part of reproductive organization. [González et al. 2023] [Rosas-Reinhold et al. 2021]

After successful fertilization, the embryo develops and the ovule becomes a seed. Tissues surrounding the seeds participate in fruit formation. Each stage may depend on parental compatibility, plant condition and environmental conditions. Pollen application or initial ovary enlargement alone does not guarantee mature, germinable seeds. [González et al. 2023] [Rosas-Reinhold et al. 2021]

In self-incompatible plants, a second offset of the same mother plant usually remains a member of the same clone. Transferring pollen between such offsets is not equivalent to selecting an independent partner. For planned propagation, the propagation method and provenance of both plants matter, not simply the number of pots. [González et al. 2023] [Rosas-Reinhold et al. 2021]

Observing flowers in a collection

Documentation should include the whole flower, the surface of its lower portion and the arrangement of perianth, stamens and stigma. Opening time, flower longevity and changes in appearance over successive days can be recorded separately. These observations help compare individuals, but colour and shape alone do not establish which animal pollinator ensures reproductive success in the wild. [Rosas-Reinhold et al. 2021] [Fleischer and Schütz 1978]

During pollination, label the mother plant, pollen donor, date and specific flower. The subsequent record should include fruit ripening, seed number and germination results. This distinguishes an assumption of successful crossing from documented production of offspring. [Rosas-Reinhold et al. 2021] [Fleischer and Schütz 1978]

A full flower description thus combines morphology, development and function. Collector observations provide a starting point, anatomy explains tissue organization and developmental studies reconstruct how that organization arose. None of these levels replaces the others. [Rosas-Reinhold et al. 2021] [Fleischer and Schütz 1978]

Hand pollination and crossing records

In the diagram, pollen is transferred from the anther to the stigma. This is not yet fertilization. The pollen grain must germinate and the tube reach the ovule. Transferring a large quantity of pollen does not guarantee compatibility or seed formation. Pollen maturity, stigma receptivity and partner compatibility matter. [Rosas-Reinhold et al. 2021] [Ramírez-Castro et al. 2024]

Correctly identified plants are selected for crossing. Two offsets of one clone do not become genetically different partners. Use a clean tool to collect pollen from the selected donor and apply it to the stigma of the labelled flower without damaging its surface. Protect the flower from accidental foreign pollen according to its structure and opening period. Moving from one donor to another requires a separate or appropriately cleaned tool. [Fleischer and Schütz 1978, pp. 163–169]

The record gives the mother plant first, followed by × and the pollen donor, date and collection numbers. A fruit alone does not demonstrate hybrid origin. Self-pollination, foreign pollen contamination and, in certain taxa, other routes of seed formation are possible. A breeding experiment should include appropriate control flowers. The name of an expected cross does not replace confirmation of its parentage. [Rosas-Reinhold et al. 2021] [Ramírez-Castro et al. 2024] [Fleischer and Schütz 1978, pp. 163–169]

A flower enclosed by a short shoot

In Calymmanthium substerile, the true bud develops inside an extensive shoot covering with areoles and spines. Just before the perianth opens, this covering opens at its apex. This particular relationship between flower and pericarpel is developed in a separate account. [de Vos et al. 2025]

Separation of sexual functions in cholla

In Cylindropuntia wolfii, organ fertility divides flowers into functionally male and female forms. These differences accompany variation in colour, average size and insect visits. A separate article links these observations to results of population genetic analysis. [Ramadoss et al. 2023][Ramadoss et al. 2025]

Temperature between initiation and bud emergence

Dragon fruit floral buds pass through stages on which light and temperature act differently. In Chu and Chang’s experiment, the white-fleshed Selenicereus undatus cultivar ‘VN White’ received a fourteen-hour day. Under cooler conditions, buds developed more slowly and remained beneath the areole. Higher temperature accelerated their emergence at the surface. The study therefore distinguishes the signal associated with day length from subsequent primordium growth under particular thermal conditions. [Chu and Chang 2020, pp. 2–6]