IAWA Journal, Vol. 21 (4), 2000: 457–462 OCCURRENCE OF PERFORATED RAY CELLS IN GENERA OF PACHYCEREEAE (CACTACEAE) by Teresa Terrazas Programa de Botánica, Colegio de Postgraduados, Montecillo, Estado de México 56230, Mexico SUMMARY Perforated ray cells are reported for the first time for 16 species of seven genera of Pachycereeae. The perforated ray cells are shorter than vessel elements and have simple perforation plates and small alternate intervas- cular pits. Among Cactaceae these specialised cells seem to be related with splitting of the tall rays. Key words: Perforated ray cells, Pachycereeae, columnar cacti, Cacta- ceae. INTRODUCTION Perforated ray cells, or ray cells of the same dimensions or larger than the adjacent cells, but with perforations, have been recorded in a number of unrelated families (Chalk & Chattaway 1933; Carlquist 1988). Carlquist (1961) noted that perforated ray cells are derived from ray initials and possess either simple or scalariform perfo- ration plates, matching the other vessel elements in the species. McLean and Richardson (1973) suggested that perforated ray cells may function as a part of the vertical con- ducting system of the xylem and named them vascular ray cells. Although perforated ray cells are commonly confined to exclusively uniseriate rays or uniseriate exten- sions of tall multiseriate rays (Carlquist 1988; Nagai et al. 1994; Eom & Chung 1996), they occasionally occur in the multiseriate portion of wide rays (Bottoso & Gomes 1982; Dayal et al. 1984; Rao et al. 1984). The diagnostic value of these cells has been questioned by several authors (Rudall 1985; Carlquist 1988; IAWA Committee 1989). However, based on ontogeny Otegui (1994) argued in favour of their diagnostic value among species of Rapanea (Myrsinaceae). Apart from Srivastava and Bailey (1962) who showed a conversion of ray initials into fusiform initials in the leaf-bearing Cactaceae, there are no accounts of perfo- rated ray cells in the Cactaceae. Wood features of columnar cacti belonging to differ- ent tribes of subfamily Cactoideae have been described (Gibson 1973; Mauseth 1996; Mauseth et al. 1998), but perforated ray cells have not been reported. Pachycereeae is a tribe in which most of the large tree-like or tall columnar Mexican species of cacti belong (Barthlott & Hunt 1993). The tribe is mostly endemic to Mexico and contains ten genera and nearly 50 species, which are commonly distributed in seasonal de- ciduous forest and desertscrub or thornscrub. This paper summarises the general wood features and reports the occurrence of perforated ray cells in species of Pachycereeae. Downloaded from Brill.com10/01/2021 06:09:16AM via free access 458 IAWA Journal, Vol. 21 (4), 2000 Terrazas — Perforated ray cells in Cactaceae 459 Table 1. Species studied. Species No. Locality Samples Cephalocereus columna-trajani (Karw.) K. Schum. 2 Tehuacan Valley, Puebla Myrtillocactus geometrizans (Mart.) Console 2 Tehuacan Valley, Puebla Neobuxbaumia macrocephala (F.A.C. Weber) E.Y. Dawson. 1 Tehuacan Valley, Puebla mezcalaensis (Bravo) Backeb. 1 Zopilote Canyon, Guerrero tetetzo (F.A.C. Weber) Backeb. 2 Tehuacan Valley, Puebla Pachycereus grandis Rose 2 Tehuacan Valley, Puebla hollianus (F.A.C. Weber) Buxb. 2 Tehuacan Valley, Puebla pecten-aboriginum (Engelm.) Britton & Rose 3 Sonoran desert, Sonora pringlei (S. Watson) Britton & Rose 1 Sonoran desert, Sonora tepamo Gamma & Arias 2 Infiernillo dam. Michoacan Polaskia chichipe (Rol.-Goss.) Backeb. 1 Tehuacan Valley, Puebla Rathbunia alamosensis (Runge) Britton & Rose 3 Sonoran desert, Sonora Stenocereus chrysocarpus Sánchez-Mej. 2 Infiernillo dam, Michoacan fricii Sánchez-Mej. 3 Infiernillo dam, Michoacan queretaroensis (F.A.C. Weber) Buxb. 2 Queretaro Valley, Queretaro stellatus (Pfeiff.) Riccob. 1 Tehuacan Valley, Puebla MATERIALS AND METHODS Wood samples of 16 species were mainly collected in the Tehuacan Valley, Balsas depression, and the Sonoran desert (Table 1). Wood pieces, which represent a com- plete fascicular area including pith to cambium, were taken from the main stem, about 10–15 cm from the ground in most samples. All samples were fixed immediately in FAA solution (Johansen 1940), then washed in water and stored in a glycerine-etha- nol-water (1 : 1 : 1) solution until sectioning. Sections were cut with a sliding micro- tome at 20–25 μm in thickness. Apart from cross and radial sections, serial tangential sections were cut at intervals of nearly 1–2 mm. Sections were double-stained with safranin-fast green and mounted with synthetic resin. Quantitative data were based on 25 measurements per sample obtained with an image analyser (Media Cybernetics 1997). RESULTS Wood of examined species is characterised by being diffuse-porous with indistinct growth rings. Vessels are mostly solitary with few short groups of 2–5. Tangential vessel diameter varies from 57 ± 7 μm in Neobuxbaumia mezcalaensis to 88 ± 17 μm in Polaskia chichipe. Perforation plates are simple, mostly transverse. Intervascular pitting varies from alternate and pseudoscalariform to scalariform. Tyloses and de- posits are absent. Libriform fibres are mostly septate with conspicuous simple pits on the radial walls. Parenchyma is scanty and vasicentric, in strands of two cells and de- Downloaded from Brill.com10/01/2021 06:09:16AM via free access 458 IAWA Journal, Vol. 21 (4), 2000 Terrazas — Perforated ray cells in Cactaceae 459 posits are absent. Rays are heterogeneous with primary and secondary rays clearly distinctive in most species. Primary rays are those that originate in the interfascicular regions, whereas secondary ones developed later in the fascicular or interfascicular region after the vascular cambium has been established. Uniseriate rays are absent. Fig. 1–6. Perforated ray cells in Pachycereeae. – 1: Stenocereus queretaroensis (TS). – 2: Neobuxbaumia mezcalaensis (RLS). – 3: Pachycereus grandis (TLS). – 4: Stenocereus fricii (TLS). – 5: Pachycereus hollianus (RLS). – 6: Pachycereus tepamo (TLS). — Scale bars: 1, 4, 6 = 100 μm; 2, 3 = 50 μm; 5 = 30 μm. Downloaded from Brill.com10/01/2021 06:09:16AM via free access 460 IAWA Journal, Vol. 21 (4), 2000 Terrazas — Perforated ray cells in Cactaceae 461 Multiseriate rays vary from 8–15 cells wide, with means of 163 ± 37 μm in Myrtillo- cactus geometrizans to 399 ± 70 μm in Pachycereus hollianus. There are two distinct sizes of total ray height. The primary rays are the tallest with means of 4.21 ± 1.31 mm in Stenocereus stellatus to 12.53 ± 1.14 mm in Pachycereus pringlei. The secondary rays are shorter with means of 1.09 ± 0.53 mm in Myrtillocactus geometrizans to 2.91 ± 0.30 mm in Pachycereus hollianus. Sometimes rays were too tall for both ends to be included in the sample blocks (15–20 mm). Rays have mostly short upright and square cells, but a few procumbent cells occur in the central portion of the rays of a few species. Abundant round starch grains fill ray cells in most species studied. Perforated ray cells occur infrequently, but were present in all samples studied (Fig. 1–6). They are short and wide with simple perforation plates and small bordered intervascular pits. Perforated ray cells differentiate as a single cell in the ray centre or near one of the two margins. However, as new perforated cells differentiate, they con- stitute a row longer than the ray width, extending from the ray and connecting the two vessels that run along both sides of the ray (Fig. 4, 6). With this connection they be- come part of the axial system. In cross sections, they are not conspicuous, but can be observed when segments of an individual vessel are separated by a perforated cell and the simple pits of the ray cells are not observed (Fig. 1). In tangential view the perforation plates are seen in the tangential wall of the ray cell. DISCUSSION In the Pachycereeae perforated ray cells are characteristic of tall rays that have sub- divided into smaller segments, as occurs in other taxa (Carlquist 1988; Otegui 1994). They appear to be a common feature in most genera of Cactoideae with tall rays, but have probably been unreported because of their irregular occurrence. These cells mostly occur in the centre of wide rays, in a manner similar to some genera of Santalaceae, Annonaceae and Loganiaceae. They are not present in narrow rays (Botosso & Gomes 1982; Dayal et al. 1984; Rao et al. 1984). This condition has also been observed in other members of Cactoideae with shorter rays, such as species of the genus Acan- thocereus and Peniocereus (Terrazas, unpubl. data). In rays of both sizes, perforated ray cells that join with vessels may be a way in which the conductive system keeps its efficiency and contact points in the hydraulic system (McLean & Richardson 1973). Since the interfascicular regions of Cephalo- cereus, Neobuxbaumia, and Pachycereus only possess vessels and fibres in the older xylem, perforated ray cells may help to interconnect the conducting system in the fas- cicular region where rays are abundant and tall. As a result, the perforated ray cells in Pachycereeae are likely one of the many factors that contribute to maintaining the succulence needed to survive in the harsh environments. Rays in the subfamily Cactoideae are highly variable (Gibson 1973). In most spe- cies with fibrous woods, as in Pachycereeae, ray cells are lignified. These features are shared with Pereskia and are considered relictual for members of the Cactoideae (Mauseth & Landrum 1997; Mauseth et al. 1998). Wilcoxia species with fibrous wood are rayless (Gibson 1973; Loza-Cornejo & Terrazas 1996). Raylessness probably rep- Downloaded from Brill.com10/01/2021 06:09:16AM via free access 460 IAWA Journal, Vol. 21 (4), 2000 Terrazas — Perforated ray cells in Cactaceae 461 resents a different line of specialisation. Furthermore, the more derived members of Cactoideae may possess unlignified rays (Gibson 1973; Mauseth et al. 1998) with wide-band tracheid wood (Mauseth et al. 1995; Mauseth et al. 1998). In tangential and radial sections of Echinocereus, Ariocarpus, Astrophytum, Ferocactus, Mammil- laria, and Thelocactus, which have wide-band tracheid wood, no perforated ray cells were observed in the unlignified rays (Terrazas, unpubl.