The vascular cambium of climbing plants is usually located around the circumference of the stem, and if the activity of the cambium is not continuous, some of the circumferential regions produce tissue that is very different from the adjacent areas. Such irregular growth may be associated with the cambium or with morphogenetic factors derived from the cambium that affect cell activity [1]. Under normal conditions, each cambium produces xylem centripetally and phloem centrifugally, but both types of tissue have large amounts of parenchyma cells. All parenchyma cells in climbing plants can be redifferentiated into meristematic cells, which may give rise to vascular bundles, cork cambia, dilatation tissue, or adventitious buds [1]. The stem cross-section generally changes from a regular circular to an irregular shape after secondary growth, owing to the uneven deposition of secondary xylem and the eccentricity of successive cambia [2]. As the stem twines around a support, cambial activity increases the amount of xylem and phloem on the opposite side of the stem [3].

Cambial variants are divided into two types: those that originate from a single cambium and those that originate from multiple cambia [4]. Within Convolvulaceae, several cambial variants have been reported, such as successive cambia, interxylary phloem, xylem parts dispersed by parenchyma, and intraxylary phloem [4] [5] [6] [7] [8]. The successive cambia are divided into concentric bands and non-concentric bands [9]. The activity of concentric bands produces vascular bundles of the same width, and parenchyma cells with alternating, well-defined xylem and phloem rings; the pith is usually located in the center. The activity of non-concentric bands produces an asymmetric band in different directional successive cambia, leading to intensified development of one to three sides of the stem and an off-center pith [10].

The genus Argyreia develops successive cambia [11], medullary bundles, interxylary phloem, and intraxylary phloem [12] [13]. The anatomy of several species of Argyreia has been studied, including the structure of the medullary bundles in A. roxburghii (Wall.) Arn. ex Choisy [12] and the tri-lobed stem and successive cambia in A. hookeri C. B. Clarke [8].

The genus Ipomoea has successive cambia, interxylary phloem, and fibrous woody parts dispersed by parenchyma proliferation [11]. In an early stage of primary growth, some Ipomoea species stems form primary xylem [14]. Under normal developmental conditions, the vascular bundles in the genus Ipomoea usually develop precisely in their positions. Initially, the secondary growth of Ipomoea is normal, producing both xylem and phloem. This is followed by the activity of additional cambia, each forming around a single vascular bundle or group of vascular bundles, producing both xylem and phloem.

Lowell and Lucansky [15] reported that the primary growth of Ipomoea hederifolia L. comprises a bicollateral bundle stage and a cambium-like meristem stage, whereas the secondary growth consists of a normal cambial stage, an anomalous stage, and a supernumerary cambial stage. Rajput et al. [16] indicated that the discontinuous segments of the secondary xylem in the second ring in I. hederifolia showed xylem with reverse orientation, producing secondary xylem centrifugally and secondary phloem centripetally. Patil et al. [17] also showed that one of the successive cambial layers develops anomalous cambial arcs/segments with reverse orientation in I. hederifolia. The stem thickness of Ipomoea obscura (L.) Ker Gawl. and Ipomoea pes-caprae (L.) R. Br. in cross-section consist of three to four layers of secondary xylem and alternating phloem rings, giving rise to multiple/successive cambia [2] [18]. The stem of I. quamoclit L. subsequently develops lobed, concentric, and diffuse (foraminate) anomalous secondary growth [19]; and I. turbianata Lag. forms internal cambium and intraxylary phloem [7].

Convolvulaceae comprises approximately 53 genera and 1660 species distributed worldwide [20]. Of these, 14 genera and 44 species of Convolvulaceae, mostly lianas, have been recorded in Taiwan [21]. From 2000 to 2017, 14 species were published in Taiwan, including one new species of the genus Argyreia [22] and 13 naturalized species, including two species of the genus Cuscuta [23], five species of the genus Ipomoea [24] [25] [26] [27] [28], one species of the genus Evolvulus [29], four species of the genus Merremia [30] [31] [32] [33], and one species of the genus Jacquemontia [34]. Thus, there are approximately 14 genera and 58 species of Convolvulaceae in Taiwan.

Multiple types of cambial variants are found within Convolvulaceae, but the growth patterns of the stem cross-sections of many members of Convolvulaceae in Taiwan have not been described. Various stem structures and identifications have been reported in the Menispermaceae and Fabaceae families in Taiwan [35] [36]. In the present study, we examined the stem cross-sections of Convolvulaceae vines in Taiwan, and how the cambial variants combined with successive cambia and intraxylary phloem. As cambial variations constitute an extremely diverse morphology, the present study attempts to 1) provide detailed photographs of the features discussed and 2) provide a bracketed key based on the anatomical characteristics of the stems to facilitate the identification of irregular cambial activity in the Convolvulaceae family in Taiwan.

In the present study, the stem of the genus Argyreia was usually round or elliptical in cross-section and the secondary xylem and secondary phloem usually formed concentric rings without rays. Parenchyma proliferation was not obvious (Figure 1). In contrast, the stem of the genus Ipomoea is irregular in cross-section, the secondary cambium rings are arcs/segments-like, and parenchyma proliferation is obvious (Figure 2(b) , Figure 2(c), Figure 2(f), Figure 3(a), Figure 3(c), Figure 4(a)). The texture of the stems of Ipomoea is quite soft with irregularly distributed patches of thin-walled parenchyma, but there are almost no irregular patches of parenchyma in Argyreia. The vegetative morphologies of Argyreia and Ipomoea are quite similar, resulting in some species being erroneously described within the genus Ipomoea or Argyreia [41]. The cambial variations in their stems could be used in the debate around the species delimitation of these two genera. Vasicentric or aliform-confluent paratracheal parenchyma cells around the vessels of Argyreia and Ipomoea have been previously recorded [42], and the presence of vasicentric paratracheal parenchyma was confirmed around the vessels of the 17 species in this study.

Metaxylem elements (also called the primary xylem) differentiated from the cambium-like meristem on the primary growth were observed in these 17 species from Taiwan, and the results confirmed the metaxylem characteristics described in I. hederifolia by Lowell and Lucansky [15]. During secondary growth, the anomalous cambial stage produces the secondary xylem and secondary phloem. If the formation of secondary xylem is greater in one region than in another, it resulted in a grooved stem [16]. In this study, the stem cross-sections of I. hederifolia had two deep lobes of successive cambia. The characteristics of these lobed stems have not been described in previous reports. They might be related to stem size and the supernumerary cambial stage, and warrant further studies.

The asymmetry in the cross-sections of vines is often related to the supports used by the twining vines and results from increased tissue production in the vascular bundles. The site of the increased cambial activity is often located on the side of the stem that is not in contact with the support, e.g., in I. quamoclit [19]. In the present study, significant secondary xylem and phloem growth was observed in I. alba, I. cairica, I. indica, and I. triloba on the side of the stem opposite the support (Figure 2(b), Figure 2(c), Figure 2(f), Figure 3(e), Figure 3(f)), indicating differential cambial activity.

The eccentricity of the stem cross-sections is owing to irregular activity in the cambium causing uneven deposition of the secondary xylem. The directional intensified development of successive cambium was observed in four species, I. hederifolia (Figure 2(e)), I. obscura (Figure 3(c)), I. triloba (Figure 3(f)), and Merremia gemella (Figure 4(b)), along with clear, alternating xylem and phloem rings and functionally slow cambium activity on the side of the stem opposite the support. Intensified development of successive cambia can also increase stem thickness and alter the form of the pith, making it irregular, as confirmed in all the Ipomoea species, especially in I. alba, I. cairica, I. indica, I. obscura, and I. triloba.

The characteristics of the small arcs/segments of internal cambium in I. hederifolia were developed during the supernumerary cambial stage of secondary growth, as described by Lowell and Lucansky [15], Patil et al. [17], and Rajput et al. [18]. In this study, the diagnostic characteristics of the cambium arc/segment rings were present in D. tuberosa, E. henryi, I. cairica, I. obscura, I. pes-caprae subsp. brasiliensis, I. triloba, and Operculina turpethum. The species E. henryi developed more layers as adjacent segments separated by xylem rays, which would form a dissected xylem [6] in larger stems.

The supernumerary cambial stage is a common developmental stage in Ipomoea [19]. The supernumerary cambial refers to the presence of many successive rings. The stems of I. nil (Figure 3(b)) and I. violacea (Figure 4(a)) had only one layer of secondary xylem, whereas the stems of the remaining eight Ipomoea species possessed two to four layers of continuous xylem rings and alternating phloem rings. The stem development of the I. nil and I. violacea samples might not yet have reached the supernumerary cambial stage.

Cambial variants have been used as diagnostic characteristics for the identification of climbing plants in different families and genera, such as Aristolochiaceae, Menispermaceae, and Vitaceae. A variety of cambial variances developed in the Convolvulaceae family (Figures 1-4), as well as some climbing plants with regular secondary growth (Figures 1(a)-(f), Figure 2(a), Figure 3(d), Figure 4(a), Figure 4(c)). The development of the cambial variations in the secondary xylem rings did indeed differ depending on stem size (Figure 1(f), Figure 2(a)). Collecting all the different stem sizes for comparison each other are needed, and what another causes influence cambial variations in climbing plants warrant further investigation. The cambial variants of Convolvulaceae include species with unequal deposition of xylem, lobed stem formation (Figure 2(d), Figure 2(e), Figure 3(e), Figure 3(f), Figure 4(b)) that may be asymmetrical or symmetrical, intraxylary phloem, cambial arc/segment rings, and dissected xylems, all caused by the parenchyma proliferation as well as succession cambia. These variants may also be found in different combinations. Whether or not these variances have developmental relationships warrants further study.

Studies on the cambial variants of climbing plants in the family Convolvulaceae in Taiwan are scarce. In this study, we examined species in which cambial variants in the stem were combined with successive cambia and intraxylary phloem in Convolvulaceae from Taiwan. The experimental results showed that the stems of the two Argyreia species are round or elliptical and the secondary xylem and phloem rings are continuous. The cambial variants of Ipomoea species, except I. nil and I. violacea, include irregular conformations, successive cambia, and xylem parts dispersed by rays into segments. The first secondary xylem in E. henryi, I. nil, I. pes-caprae subsp. brasiliensis, and I. triloba is divided by numerous rays, and those of the remaining species are divided by three to five rays. The vascular bundles of E. henryi continue division and develop more layers which become adjacent segments separated by xylem rays, forming a dissected xylem. The secondary growth of D. tuberosa and O. turpethum results in only two layers of successive cambia and the second secondary xylem is divided by rays into arcs/segments. Parenchyma proliferation and mucilaginous canals distributed in the inner cortex develop noticeably in I. alba, I. cairica, and I. indica. Deeply lobed stems with directional successive cambia form irregular shapes in I. hederifolia, I. obscura, I. triloba, and M. gemella, and shallowly lobed stems form triangular shapes in I. littoralis and Stictocardia tilifolia. The successive cambia of I. pes-caprae subsp. brasiliensis forms concentric continuous bands. The xylem vessels dispersed throughout the stem are diffuse-porous and the vessels are surrounded by vasicentric paratracheal parenchyma in all the studied species. The results presented herein provide an important basis for further morphological studies on the cambial variances within this family.

We truly appreciate the assistance of the staff of the PPI Herbarium at the National Pingtung University of Science and Technology for providing fresh Convolvulaceae stem specimens in collections and photos. The authors are extremely grateful to two reviewers whose suggestions improved this manuscript.

The authors declare no conflicts of interest regarding the publication of this paper.

Yang, S.-Z., Chen, P.-H. and Chen, C.-F. (2020) Cambial Variants Combine Successive Cambia and Intraxylary Phloem in Convolvulaceae in Taiwan. American Journal of Plant Sciences, 11, 437-453. https://doi.org/10.4236/ajps.2020.113032