Morphological characters

1. Growth form — (0) depressed-globose, (1) globose, (2) cylindric, (3) columnar (Fig. 2). Based on individual height and diameter, four growth forms are recognized for members of Cacteae (Vázquez-Sánchez & al. 2012). When plants are wider than they are tall, with a compressed vertical axis, the plant is characterized as a depressed-globose growth form, as in the cases of Aztekium ritteri and Lophophora williamsii. If the stem has approximately the same height as its width, as in the case of Thelocactus conothelos, the growth form is referred to as globose. When the stems are characterized by a height greater than but not exceeding twice the diameter, the growth form is considered cylindric, which is the most common form for the tribe. When the plant has a height exceeding twice its diameter, we refer to it as the columnar growth form, as in the case of Ferocactus peninsulae.

2. Podaria arrangement — (0) tubercles, (1) tubercled ribs, (2) ribs (Fig. 3). During seedling development in Cactoideae, the apical meristem forms podaria or tubercles (Buxbaum 1950; Bravo-Hollis 1978; Loza-Cornejo & Terrazas 2011). In seedling development, tubercles are organized in spiralled acropetal series, and this arrangement has been found to conform to the Fibonacci sequence (Gibson & Nobel 1986). Sometimes tubercles are arranged in longitudinal series, but they do not fuse together (Bravo-Hollis 1978; Gibson & Nobel 1986). In the tribe Cacteae, species of smaller size are characterized by the presence of tubercles, as in the cases of Mammillaria and Turbinicarpus. These tubercles can also be arranged in longitudinal series exhibiting almost complete fusion, forming a rib that is named a tubercled rib, as in the cases of Ferocactus gracilis and Thelocactus bicolor. When the podaria are perfectly fused and arranged in a vertical orthostatic series, ribs are generated; the fusion of the apical meristem's podaria is no longer evident in plants such as Echinocactus platyacanthus and Geohintonia mexicana.

Anatomical characters

3. Wood — (0) without fibres, (1) fibres rare, (2) fibres forming patches, (3) dimorphic (Fig. 4). Wood without fibres is composed exclusively of vessels and wide-band tracheids (WBT). A WBT is a specialized type of tracheid characterized by the presence of annular or helical thickenings in its secondary wall. They are short, wide and spindle-shaped, with a secondary wall band covering a small part of the primary wall (Mauseth & al. 1995). When the wood is retained with a WBT matrix near the vascular cambium, vessels are embedded in it, and each vessel element shows a similar pattern of secondary walls as the WBT. This wood without fibres is characteristic of small-sized plants in genera such as Ariocarpus, Cumarinia and Turbinicarpus. When fibres are present near the vascular cambium, they can be scarce, as in the cases of Ferocactus histrix and Thelocactus conothelos; they can form patches as seen in some species of Astrophytum and Ferocactus robustus or they can exhibit dimorphic wood (i.e. they produce one type of wood when young and a completely different type as adults, as in Geohintonia, Echinocactus platyacanthus, Echinomastus mariposensis and several Ferocactus species. Only in the dimorphic wood do fibres appear in all fascicles of the vascular cylinder and vessel elements show pseudoscalariform or alternate intervascular pitting.

4. Dilated rays — (0) absent, (1) present throughout, (2) present only in region close to pith (Fig. 5). Rays are parenchymal cells that extend radially (outwards) on both sides of the vascular cambium and favour radial communication from the pith to the cortex. Rays can vary in height and width, and their walls are commonly lignified (Carlquist 2001); they usually dilate in the bark due to an increase in mitotic divisions with an anticlinal or oblique orientation (Evert 2006). In Cacteae, they are rarely lignified and may not be dilated, as in Aztekium ritteri and most Ferocactus species; alternatively, they can be dilated throughout, as in Ariocarpus, Echinocactus and Epithelantha or dilated only in the region close to the pith as in some Astrophytum and Stenocactus.

5. Hypodermis cells — (0) thin-walled, (1) thick-walled (Fig. 6). The hypodermis is located under the epidermis. It usually consists of more than one layer of cells in succulent stems. Its cell walls can be thin or thick; when thick, the hypodermis is collenchymatous as in Leuchtenbergia and most Ferocactus species; in addition to being collenchymatous, the cells occur in several layers, providing rigidity to the stems (Terrazas & Mauseth 2002).

6. Cortex — (0) parenchymatous, (1) collenchymatous, (2) sclerified (Fig. 7). The cortex is commonly composed of parenchyma cells. This is the condition present in most species of Cacteae. A collenchymatous cortex is characteristic of Mammillaria candida and Neolloydia conoidea, which is differentiated from a parenchymatous cortex by a thick primary cell wall. A third state appears when the cortex has lignified cells (i.e. it has sclereids, as in species of Cochemiea, Stenocactus and Thelocactus, as well as Coryphantha cornifera).

7. Pith — (0) parenchymatous, (1) sclerified (Fig. 8). Pith consists of parenchyma with thin-walled isodiametric cells; however, in some Cochemia, Stenocactus and Thelocactus species, the pith has sclereids with lignified walls.

Fig. 2.

Parsimony reconstruction of growth form. Numbers after genus names indicate numbers of taxa studied; numbers above lines are Bayesian values. Photos show character states, colourcoordinated with states on tree. Depressed-globose: Lophophora williamsir, globose: Thelocactus conothelos; cylindric: Sclerocactus scheert; columnar: Ferocactus peninsulae.

Fig. 3.

Parsimony reconstruction of podaria arrangement. Numbers after genus names indicate numbers of taxa studied; numbers above lines are Bayesian values. Photos show character states, colour-coordinated with states on tree. Tubercles: Turbinicarpus saueri subsp. knuthianus; tubercled ribs: Ferocactus gracilis; ribs: Echinocactus platyacanthus.

Fig. 4.

Parsimony reconstruction of wood. Numbers after genus names indicate numbers of taxa studied; numbers above lines are Bayesian values. Photos show character states, colour-coordinated with states on tree. Without fibres: Ariocarpus agavoides; fibres rare (intermixed with vessels and parenchyma cells): Thelocactus conothelos; fibres forming patches: Astrophytum ornatum; dimorphic: Ferocatus pilosus. Scale bars = 100 µm, except dimorphic = 300 µm; r = ray; black or white arrows indicate fibres; bracket indicates change from wide-band tracheids to fibres.

Fig. 5.

Parsimony reconstruction of dilated rays. Numbers after genus names indicate numbers of taxa studied; numbers above lines are Bayesian values. Photos show character states, colour-coordinated with states on tree. Absent: Aztekium ritteri; present throughout: Epithelantha micromeris; present only in region close to pith: Astrophytum capricorne. Scale bars = 100 µm, except absent = 300 µm; r = ray.

Fig. 6.

Parsimony reconstruction of hypodermis cells. Numbers after genus names indicate numbers of taxa studied; numbers above lines are Bayesian values. Photos show character states, colour-coordinated with states on tree. Thin-walled: Cumarinia odorata; thick-walled: Leuchtenbergia principis. Scale bars = 100 µm; ^* = thin-walled cells of hypodermis.

Fig. 7.

Parsimony reconstruction of cortex. Numbers after genus names indicate numbers of taxa studied; numbers above lines are Bayesian values. Photos show character states, colour-coordinated with states on tree. Parenchymatous: Geohintonia mexicana; collenchymatous: Neolloydia conoidea; sclerified: Stenocactus obvallatus. Scale bars = 300 µm.

Fig. 8.

Parsimony reconstruction of pith. Numbers after genus names indicate numbers of taxa studied; numbers above lines are Bayesian values. Photos show character states, colourcoordinated with states on tree. Parenchymatous: Coryphantha erecta; sclerified: Stenocactus phyllacanthus. Scale bars = 300 µm.

Character evolution

Character optimization for the seven selected characters is shown in Fig. 2–8. The ancestral growth form is globose. A shift towards cylindric and columnar was found in the genera Echinocactus and Astrophytum. The cylindric growth form is found in most members of Cacteae (Fig. 2). Another typical feature of Cacteae, the presence of tubercles, is the most common condition for Cacteae stems. Character reconstruction revealed that the presence of ribs is the most ancestral condition of the tribe and has evolved repeatedly in independent events (Fig. 3). Tubercled ribs predominate with different origins in the Ferocactus clade, where there are also shifts towards more sclerified tissues.

The presence of WBT in the wood has frequently been used as a diagnostic character for species in the tribe. However, the presence of these cells is not a synapomorphy of the tribe, although it is the most ancestral condition (Fig. 4). The presence of rare or scarce and patch-forming fibres has appeared repeatedly in independent events. Dimorphic wood appeared independently in more than eight events from non-fibrous wood. Non-dilated rays are the ancestral character state of the tribe (Fig. 5). In the Echinocactus—Astrophytum clade, rays are dilated along the whole course, and the same condition is independently present in other clades. Ray dilation close to the pith is an uncommon trait and originated in at least five independent events.

Collenchymatous hypodermis is the predominant character state in the tribe (Fig. 6). Non-collenchymatous hypodermis was independently acquired more than five times. Parenchymatous cortex is the ancestral character state of the tribe (Fig. 7); the presence of collenchymatous cortex is an uncommon condition that has appeared twice independently, whereas sclerified cortex has been acquired in several independent events. The unlignified pith is the ancestral character state of the tribe. Lignified pith has appeared in seven independent events in some species (Fig. 8).

Growth form

In recent years, interest in the reconstruction of growth form evolution in succulent plants, especially in the family Cactaceae, has increased (Hernández-Hernández & al. 2011). However, it is evident that there is no general consensus regarding how to name the different growth forms that exist in the family ( Vázquez- Sánchez & al. 2012). In the case of Cacteae, where stem construction is relatively simple, it is necessary to perform detailed field observations to avoid mistakes. The growth form of Cacteae has been described mainly as globose and cylindric (Buxbaum 1950; Bravo-Hollis 1978; Anderson 2001). In this analysis, the earliest divergent clade is characterized by the globose growth form with ribs as the earliest diversified characters in the tribe. Butterworth & al. (2002) and Hernández-Hernández & al. (2011) also inferred that the ancestor of Cacteae was globose with ribs. The predominance of ribs in Geohintonia and in the Echinocactus-Astrophytum clade with cylindric and depressed-globose growth forms was the state prior to the significant radiation of Cactoideae, according to Arakaki & al. (2011). One anatomical trait of these early-derived species is the occurrence of a thick-walled epidermis and hypodermis that provides mechanical support. These species withstand marked aridity, which began in the mid-Miocene period.

Species with depressed-globose, globose and cylindric growth forms with tubercles subsequently predominated in Cacteae. These tubercled growth forms are distinguished by having more flexible tissues with thin cell walls, as in Coryphantha or Mammillaria species (Terrazas & al. 2016); they are very soft to the touch. When tubercles are arranged spirally and gaps between them are present, the vertical compression capacity may favour some biomechanical properties. Because tubercles are so flexible, they are unable to maintain an erect columnar stem (Mauseth 2006) and only appear in species of small size, unless the plant is pendent, as in some cliff-dwelling Mammillaria species. These morphological and anatomical differences were likely promoted by the development of more tropical temperature conditions, which were the result of the uprising of the Trans-Mexican Volcanic Belt from the mid-Miocene to the Holocene (Ferrusquia-Villafranca 2007). Contrasting with the globose and depressed-globose growth forms that have reappeared recurrently in Cacteae, the columnar growth form in the tribe appears in species mainly from the Sonoran desert in the Baja California Peninsula. Because these shoots can develop as comparatively more fully integrated entities over time, they are often able to reach larger dimensions — in some cases, more than two metres in height. Only Echinocactus platyacanthus and Ferocactus pilosus from the Chihuahuan desert display this columnar growth form, in which the succulent tissue percentage can exceed 90 % (Vázquez-Sánchez & Terrazas 2011). With no exceptions, columnar species show ribs or tubercled ribs, functioning as stem-supporting columns, as suggested by Reyes-Rivera & al. (2015). Another trait of these species with a columnar growth form that could contribute to the reinforcement of the mechanical support is the presence of a collenchymatous hypodermis, which provides rigidity (Esau 1976; Terrazas & Mauseth 2002).

Secondary xylem

Wood with WBT is the ancestral condition for the xylem type in the tribe. This is a character shared with Blossfeldia liliputana, which exclusively has WBT (Mauseth 2006). It has been argued that virtually all Cactaceae with globose or depressed-globose growth forms have tubercles and a non-fibrous wood, in which vessels are embedded in a matrix of WBT or parenchyma (Gibson 1973; Mauseth 2006; Mauseth & Plemons-Rodríguez 1998; Mauseth & al. 1998; Terrazas & Mauseth 2002). However, in Cacteae, there are exceptions, such as Geohintonia mexicana, which has a globose growth form with ribs and dimorphic wood, and Echinomastus mariposensis, which has a cylindric growth form with tubercles, both species of less than 10 cm tall. What makes vascular cambium in Cacteae shift from poorly lignified conductive and supporting cells to more lignified ones appears to be not only related to size.

According to Mauseth (2006), one of the advantages of having WBT in species with short, tubercled stems is that these allow the whole stem to shorten or enlarge in response to changes in water content. Additionally, the presence of tubercles is associated with the maintenance of WBT in the wood of adult individuals. The proportion of primary walls in the WBT and vessel elements, as well as the absence of other lignified cell types, such as fibres or rays, favours the vertical compression capacity of the vascular tissue during drought periods (Vázquez-Sánchez & Terrazas 2011).

In Cacteae, dimorphic wood is associated with plants reaching a greater height, with the exception of Geohintonia and Echinomastus mariposensis, as mentioned above. The shift from WBT to fibres as the predominant wood cell type is necessary to provide taller stems with better mechanical support. Niklas (1992) mentioned that the rigidity and larger size of succulent stems may be due to the tension created by the epidermis and collenchymatous hypodermis across the circumference. Based on this assertion, Cornejo & Simpson (1997) used Ferocactus wislizeni as a model to conclude that, in this species, which reaches two metres in height, support is derived only from the rigidity of the fundamental tissue because the plant does not produce a woody skeleton. However, in F. wislizeni, as in other Ferocactus species, there is dimorphic wood, so support is derived from a combination of the degree of succulence, a thick hypodermis and the fibres present in the wood of adult individuals, as described in the present investigation. In species with dimorphic wood, the vertical compression capacity is reduced due to the lignification of secondary walls, not only in fibres but also in vessel elements. Vessel elements switch from helical thickenings to pseudoscalariform and alternate intervascular pits (Grego-Va-lencia & al. 2015; Terrazas & al. unpubl. data). In recent studies, Reyes-Rivera & al. (2015) found that the proportion of lignin also increases with stem height (26 cm = 9.7 %, 38 cm = 28.8 %, 150 cm = 51.8 %). Fibres and vessel elements arranged in fascicles generate rigid columns (Vázquez-Sánchez & Terrazas 2011) and, together with ribs, allow the cortex to expand and contract in a radial direction without damaging the stem surface (Mauseth 2006). This is likely similar to a mechanism employed by arborescent cacti such as Myrtillocactus geometrizans or Pilosocereus pachycladus, in which the columnar Cacteae form tension wood in regions of hightension stresses (Schwager & al. 2013). As mentioned previously, the collenchymatous hypodermis and thick cortex in these species with taller stems seem to be important for mechanical support.

Wood with WBT in Glandulicactus and Thelocactus is commonly related to growth form (globose or depressed-globose) and wood with WBT is a correlated trait in species with a cylindric growth form. In the growth forms not exceeding 30 cm in height (i.e. depressed-globose, globose or cylindric), the occurrence of fibres is interpreted as reaction wood (Clair & al. 2001; Ruelle & al. 2007; Vázquez-Sánchez & Terrazas 2011), as in Coryphantha erecta and Stenocactus phyllacanthus, because it is limited to certain fascicles, and vessel elements maintain the helical secondary wall patterning. This reaction wood allows the plant to retain a vertical stem position when growing between rocks or when branching (Vázquez-Sánchez & Terrazas 2011). In addition, in species exhibiting wood with WBT and scarce or patchforming fibres, secondary growth remains limited. It is therefore likely that the broad parenchymatous cortex itself takes on the role of mechanical support (Mauseth 2006; Schwager & al. 2013) or lignification in the cortex or pith, or both, as occurs in Stenocactus and Thelocactus species. What drives the shift in the timing of secondary wall expression in the vessel elements when fibres are present in the dimorphic wood is unknown, thus attention to understand secondary wall lignification gene expression is needed (Barros & al. 2015).

Other anatomical traits

The combination of collenchymatous hypodermis, lignified pith and dilated rays in the whole course or only close to the pith was observed for depressed-globose, globose and cylindric growth forms; these are attributes independently acquired. For example, in species of Stenocactus and Thelocactus, the hypodermis is non-collenchymatous, but the cortex and pith are lignified and the rays are dilated near the pith. When pith and cortex are unlignified (e.g. Ariocarpus, Astrophytum, Aztekium, Echinocactus, Rapicactus and Turbinicarpus s.str.), rays are dilated along the whole course and the collenchymatous condition of the hypodermis is retained, allowing the plant to have more parenchyma tissue in which to store water, starch grains or crystals. In addition to ray dilation, the collenchymatous hypodermis of these genera contributes to preventing the deformation of the plants by maintaining their growth form and keeping the plant erect in the absence of lignified cells. Moreover, the occurrence of lignified cortex and pith at the base of the stems in Stenocactus and Thelocactus may contribute to maintenance of the growth form even if the members of both genera do not have wood with fibres.