Bark Morphologies
The anatomical and morphological details of stems for the 22 species investigated in this study are listed in Tables 2 and 3 and are presented in Figures 1 to 8. The stem bark contained inner bark and rhytidome (outer bark). The inner bark was located under the rhytidome and comprised the cortex and secondary phloem. Among the 22 species investigated, only C. akoensis (Fig. 1B), C. chinensis var. tatushanensis (Fig. 1E), and C. tashiroi (Fig. 7A, B) had inner bark due to their smaller stem diameters (3.8–6.0 mm) (Table 2). The bark of C. tashiroi was deep green in color and glabrous without any rhytidome in spite of large stem sizes (4.0–17.8 mm), and its stem cross-section was hexagonous. Rhytidome is comprised of successive cork and dead phloem to form dead outer bark. According to the arrangement and detachment degree of rhytidome, it can be divided into two forms: cogwheel-like rhytidome (ring bark) and continuous segment rhytidome (scale bark) (Esau 1958; Sieber and Kucera 1980; Evert 2006). The rhytidome did not last long on the bark in the 22 Clematis species. Nineteen species had wrinkled rhytidome; among them, 11 species had continuous segment rhytidome and eight species had cogwheel-like rhytidome, which peeled and teared easily. The rhytidome of C. crassifolia (Fig. 2B) was continuous segment and was the thickest (1.8–3.4 mm). That of C. lasiandra (Fig. 3E) was cogwheel-like and 0.5–2.7 mm in thickness. The remaining species were thinner than the above two species.
Cambial Variant Types
The stems of the 22 species investigated were shallowly grooved or angulated, and the stems were round (Fig. 1A, B) or hexagon and deeply grooved in shape (Figs. 2F, 6A, 6D). Only C. henryi var. morii (Fig. 3F) had an irregular conformation, forming a deeply polygonous lobe. The stem of C. formosana (Fig. 2D) was eccentric to oval or elliptical at the last stage. The Clematis stems examined generally developed axial vascular elements in segments, and the xylem were separated by wider rays, forming the xylem in plate type. This type is derived from a single cambium according to Angyalossy et al. (2012). Except for the xylem in plate type, C. gouriana subsp. lishanensis (Figs. 2D) formed discontinuous wedge-like phloem.
Variation in Vascular Elements
The secondary rays were always linear, but that of C. akoensis (Fig. 1B), C. grata (Fig. 3C), C. henryi var. henryi (Fig. 3D), C. henryi var. morii, C. pseudootophora (Fig. 6A), and C. tsugetorum (Fig. 8B) were wedge-like (Table 2, RWL column). The interfascicular cambia made the rays of C. alpina wider and wedge-like (Schweingruber et al. 2011); this character was apparent in six of the Clematis species studied. The wedge-like ray form could be referred from the average width of the primary rays. For example, C. henryi var. henryi and C. grata had the widest primary rays, with a maximum width of 668 µm and 642 µm, respectively.
Owing to the presence of thin-walled axial parenchyma in latewood adjacent to the thin-walled ray cells, wedge-shaped indentations can be seen in the interfascicular region (Carlquist 1995). The fascicular areas of stem cross-sections of Clematis species were investigated. There was an obvious indentation in the region of the wider rays (Fig. 5B), except in C. akoensis (Fig. 1B), C. chinensis var. tatushanensis (Fig. 1E), C. montana (Fig. 5C), C. psilandra (Fig. 6C), and C. tsugetorum (Fig. 8B). This is because the stem diameters of these species were too small to develop wider rays. The cortical sclerenchyma fibers of five species, C. chinensis var. chinensis (Fig. 1C, D), C. grata (Fig. 3C), C. lasiandra (Fig. 4B), C. uncinata var. okinawensis (Fig. 8C, D), and C. uncinata var. uncinata (Fig. 8E, F), were connected with the plate of sclerenchyma fibers that were embedded in the phloem rays and formed an arc outside the fascicular regions. The axial parenchyma of C. psilandra (Fig. 5E) and C. tsugetorum (Fig. 8B) were very scarce, and vessel distribution was limited to the central portions of the fascicular xylem area with growth rings. The vessels of the remaining 20 species were distributed along the edge of the fascicular areas.
Species were grouped based on the number of vascular bundles they contained, with the ‘central type’ having 12 bundles, the ‘many type’ having >12 bundles, and the ‘few type’ having <12 bundles (Smith 1928). In this study, the number of vascular bundles observed in Clematis species ranged from 6–21. Among them, three species were classified as few type (14%), 13 species were classified as many type (59%), six species were classified as central type (27%) (Table 2, VB column). Only one species, C. pseudootophora (Fig. 6A), had six vascular bundles (Table 2).
The Clematis species in this study almost all had semi-ring-porous vessels, except for C. crassifolia (Fig. 2B), C. lasiandra (Fig. 4B), C. psilandra (Fig. 6C), and C. tsugetorum (Fig. 8B), which had ring-porous vessels with distinct annual rings. None of the species had diffuse-porous vessels. Clematis parviloba subsp. bartlettii (Fig. 5E) and C. uncinata var. uncinata (Fig. 8E, F) had a primary xylem ring located around the pith.
The secondary xylem rays evolve near the periphery and split the secondary xylem by parenchyma proliferation, which can continuously increase stem diameters (Schweingruber et al. 2011). Stem cross-sections showed that eight Clematis species formed one to three secondary xylem rays within some vascular bundles [C. chinensis var. chinensis (Fig. 1D), C. formosana (Fig. 2C, D), C. gouriana subsp. lishanensis (Figs. 2E), C. grata (Fig. 3B, C), C. lasiandra (Fig. 4B), C. leschenaultiana (Fig. 5A), C. tashiroi (Fig. 7D), C. uncinata var. okinawensis (Fig. 8C, D), and C. uncinata var. uncinata (Fig. 8E, F)] (Table 2, SR column). The remaining 14 Clematis species had thick-walled cells with lignified walls.
Developmental Stages
Based on the characteristics observed in stem cross-sections of 50 samples from 22 species, samples were divided into four ontogenetic stages (Table 2). If multiple samples were collected of the same species (which was the case for 16 species), each sample was investigated separately. Therefore, these species were assigned 2–3 developmental stages.
Twelve species had dense or strand bundles of primary phloem fibers within the cortex and were identified as being in the first developmental stage (Figs. 1A, 1C, 1F, 2C, 2F, 4C, 6D, 7A, 7E, 8C, 8E),. In the second stage, the ring-like initial periderm and secondary phloem fibers were formed (Figs. 3A, 5C, 6D, 7F). In the third stage, the wider rays and many larger vessels formed centripetally; sequent periderms were produced within the secondary phloem; cork and dead phloem were detached into a continuous segment (Figs. 2B, 2D-F, 3F, 5A, 5B-C, 5D, 6C, 8B) or cogwheel-like form (Figs.1D, 3C, 4B, 6A, 6F, 8A); and few secondary xylem rays were formed. In the fourth stage, sequent periderms were produced continuously with progressively deeper layers forming rhytidomes successively, and many secondary xylem rays were formed (Figs. 1D, 2D-E, 3C, 4B, 5A, 8D, 8F).
A key to the 22 Clematis species in Taiwan, based on the characteristics of bark and vascular bundles, is provided below.
1. Vessels restricted to the central portions of the fascicular xylem area…………....2
1. Vessels unrestricted to the central portions of the fascicular xylem area………....3
2. Vascular bundles 20, pith large, white……………………..... Clematis psilandra
2. Vascular bundles 17, pith small, grey……………............…Clematis tsugetorum
3. Phloems wedge-like ……………...........…Clematis gourianasubsp. lishanensis
3. Phloems without wedge-like……………..........…………………..........…. ……...4
4. Rays wedge-like…. …………..........……………..........………..………...........….5
4. Rays without wedge-like…. …………..........……………….. …………...........….9
5. Vascular bundles 6….……………….……. …….…....Clematis pseudootophora
5. Vascular bundles > 6……………..........………………….. …….. …….............….6
6. Cortical sclerenchyma embedded in the phloem rays…………….Clematis grata
6. Cortical sclerenchyma unembedded in the phloem rays…. ……..….. …….. ……..7
7. Stem polygonous lobe……... .. ........ ……................. Clematis henryi var. morii
7. Stem round…………..........……………..........…………………......... ……..…….8
8. Bark with cortex... ………….... ………………......... ……..…Clematis akoensis
8. Bark with one to three rhytidome layers....…………..Clematis henryi var. henryi
9. Bark with cortex and vascular cylinder……………......................……….. ……...10
9. Bark with one to five rhytidome layers…. ……………….. …….......................…11
10. Vascular bundles 20……………………... Clematis chinensis var. tatushanensis
10. Vascular bundles 14-20. ………………..….…………..…...…Clematis tashiroi
11. Primary xylem ring-like………….…..Clematis parviloba subsp. bartlettii
11. Primary xylem not ring-like……………..........……………....................….12
12. Cortical sclerenchyma embedded in the phloem rays…………...................…….13
12. Cortical sclerenchyma unembedded in the phloem rays……………...........…….17
13. Vascular bundles 20-22……………………..........………...... ……..….......……14
13. Vascular bundles 12-14……………..........………………. ……................……..15
14. Rhytidome cogwheel-like………………….…Clematis chinensis var. chinensis
14. Rhytidome continuous segments. . ……………..………. ...Clematis crassifolia
15. Vessels ring-porous.............. . ……………………..…........ Clematis lasiandra
15. Vessels semi-ring-porous……………............………………........ ……....……..16
16. Pith cavity hexagon in shape..... ... …………...Clematis uncinata var. uncinata
16. Pith without cavity . ... ................................Clematis uncinata var. okinawensis
17. Vascular bundles 16-21……………..…..........……………….......... ……..…….18
17. Vascular bundles 12-19……….....................………………... …….........……...19
18. Rhytidome thickness 0.4-0.8 mm………….…...Clematis leschenaultiana
18. Rhytidome thickness 0.2-0.4 mm………………..………..Clematis meyeniana
19. Secondary xylem rays numerous.…………………............Clematis formosana
19. Secondary xylem rays few or none................………………..........…………….20
20. Rhytidome continuous segments. . ……………...………….Clematis montana
20. Rhytidome cogwheel-like……………............………..……………..........…….21
21. Vascular bundles with definite number……….….………..…..Clematis tamurae
21. Vascular bundles with indefinite number…Clematis terniflora var. garanbiensis