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Anatomy and Morphology |
2Unitat de Biologia Vegetal, Departament de Ciències Ambientals, Universitat de Girona, Campus de Montilivi s/n 17071 Girona, Spain; 3Unitat de Biologia Cel.lular, Departament de Biologia, Universitat de Girona, Campus de Montilivi s/n 17071 Girona, Spain
Received for publication June 15, 2001. Accepted for publication September 25, 2001.
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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Key Words: cotyledonary bud • cotyledonary node • lignotuber • Quercus coccifera • Quercus humilis • Quercus ilex • Quercus suber • sprout
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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, for Q. coccifera; Gratini and Amadori, 1991
; Ducrey and Turrel, 1992
; López-Soria and Castell, 1992
; Retana et al., 1992
; and Fuente, Sardans, and Rodà, 1997
, for Q. ilex; Pausas, 1997
, for Q. suber; Trabaud, 1987
, for Q. ilex and Q. suber; and Di Pasquale and Garfi, 1998
, for Q. humilis; and others).
The above-mentioned oaks species are commonly used in the reforestation of Mediterranean type forests. Although all of them are successful resprouters in their adult stage, in disturbance-prone ecosystems seedling establishment is one of the main limitations of reforestation and natural recruitment (Kozlowski, 1971
; Weltzin, Archer, and Heitschmidt, 1998
; Retana et al., 1999
). It has been reported that in holm oak forests in northeast Spain the two major disturbances affecting seedling recruitment are thinning and fire (Retana et al., 1999
). Understanding early stages of the life cycle in oaks may help to develop sylviculture strategies to improve reforestation. Thus, given that little has been written about the anatomy and morphology of young forms of the above-mentioned Mediterranean oaks, our studies focus on the analysis of morphological traits that will permit these species to resprout at the seedling stage and hence to cope with some adverse conditions. Moreover, the processes that operate during the seed, seedling, and juvenile phases are important for understanding patterns, dynamics, and succession in plant communities (Grime and Hillier, 1992
).
For cork oak we have already described the ontogeny of the cotyledonary or root-shoot transition region (Molinas and Verdaguer, 1993a, b
). In the cork oak seedlings, all the transition region is considered a true lignotuber ontogenically produced and ready for vegetative regeneration. In this species, the presence in the cotyledonary region of numerous buds placed underground and the accumulation of starch allow resprouting after damage to aboveground tissues. Furthermore, we have also studied the role of the cotyledonary node in the production of new shoots after disturbance in Q. humilis, Q. ilex, and Q. suber seedlings (Verdaguer et al., 2001
). After eliminating the shoot above the cotyledons, all three species resprouted, but when the shoot was removed from below the cotyledon level, only Q. suber seedlings survived. Based on these observations and considering the importance attributed to the cotyledonary node in the formation of anatomical structures related to sprouting (Kauppi, Rinne, and Ferm, 1987
; Del Tredici, 1992
; Graham, Walwork, and Sedgley, 1998
; Mibus and Sedgley, 2000
), we investigated the development of the cotyledonary node in Q. coccifera, Q. humilis, and Q. ilex and identified the site of lateral shoot origin in these species.
This paper focuses on the anatomy of the cotyledonary node from the mature embryo to 6-mo-old seedlings with special emphasis on the development of the buds in the axils of the cotyledons and on the bud vascular connection. The results will be discussed in comparison to embryo and seedling anatomy of Q. suber and to the presence of a lignotuber. This study will establish a base for further comparative studies on vegetative reproduction of oaks.
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MATERIALS AND METHODS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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For optical microscopy, we cut small fragments containing the embryo, root fragments, and fragments including tissue of the cotyledonary region of seedlings at different stage of development. Tissue fragments were fixed with 4% formaldehyde in phosphate-buffered saline (pH 7.5) in a vacuum at room temperature for 48 h minimum. The fragments were dehydrated through an isopropyl alcohol series and embedded in glycol-methacrylate (GMA). Sections ranging in thickness from 3 to 5 µm were cut on a rotatory microtome Autocut 1150 (Reichert-Jung, Wien, Austria) and mounted on glass slides. Routine staining was performed with toluidine blue, and to enhance the presence of carbohydrates, some sections were stained with periodic acid-Schiff (PAS) (Verdaguer and Molinas, 1997
). Specimens were photographed using an Olympus-Vanox light photomicroscope (Olympus Optical, London, UK).
For scanning electron microscopy (SEM), mature embryos carefully prepared under a dissecting microscope removing one of the cotyledonary petioles and seedling fragments containing the cotyledonary node were fixed as for optical microscopy. Fragments were dehydrated in a graded ethanol series, exchanged through amyl-acetate, and critical-point dried. The pieces were mounted on copper stubs and coated with gold. Specimens were observed using a Zeiss DSM 960A scanning electron microscope (Zeiss, Oberkochen, Germany) of the Electron Microscopy Service of the University of Girona.
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RESULTS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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4.5 mm long and showed an epicotyl with two to three pairs of leaf primordia overlapping each other and arching up over the shoot apex (Fig. 1). There were several trichomes at the margins and the abaxial surfaces (Fig. 1). At this developmental stage, a pair of rudimentary buds were present in the axils of the cotyledons (Figs. 1, 2). Longitudinal sections passing medially through both cotyledons (cotyledonary plane) also showed the shoot apex with one to two pairs of well-developed leaves and the exogenous origin, from superficial embryo tissues at the base of the epicotyl, of the first pair of cotyledonary axillary buds (Fig. 2). In transverse sections, these cotyledonary axillary buds formed oval-shaped protuberances (Fig. 3).
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3.5 mm long, were less developed, and both species had a flat or dome-shaped shoot apical meristem with two buttresses in a decussate position to the cotyledons, the first leaf primordia (Figs. 4–8). The cotyledonary axillary buds were not yet developed (Figs. 4–7). With regard to the embryo provascular tissue, the three oak species showed a similar pattern of development. In longitudinal cotyledonary sections, procambial strands could be traced from the root meristem to the petioles of the cotyledons (Figs. 2, 6, 7). In longitudinal sections passing between the cotyledons (intercotyledonary plane) procambial traces joined the two meristems (Figs. 8, 9). Despite the vascular pattern of the embryo being similar in all three oaks, serial transverse sections revealed that the number of cotyledonary vascular traces, both lateral and median, varied in the three species and even in the same species. The lateral traces arose from two bundles lying in the intercotyledonary plane, and the median traces arose from one bundle lying in the cotyledonary plane ramifying each bundle only once or not at all (Figs. 10–21). Cotyledonary traces matured earlier than bundles in the axis tissue of the provascular embryo. Just above the point where the cotyledons were attached to the embryo axis, the two cotyledonary petioles were almost fused, leaving at the center the epicotyl in which a hollow vascular cylinder was observed. Traces of the cotyledonary petioles also began to join with the vascular cylinder (Fig. 13). Below the cotyledonary insertion the tissue of the cotyledonary petioles fused to the embryo axis tissue and the vascular cylinder was interrupted by the exit of the lateral and median cotyledonary traces that enter the cotyledons (Figs. 16–18). At 0.3–0.5 mm below the cotyledonary insertion, the hypocotyl-root vascular tissue formed a hollow vascular cylinder (Figs. 19–21) that became solid at the root pole.
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8.5 mm in length and the radicle 8 mm. The hypocotyl elongated slightly or not at all and the epicotyl remained protected by the cotyledonary petioles. The epicotyl showed different stages of development of leaf primordia in the three species of Quercus (Figs. 22–24). For Q. coccifera and Q. ilex, the cotyledonary buds were not visible in any of the germinated acorns examined (Figs. 23, 24).
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3 cm long and the epicotyl was still protected by the cotyledonary petioles. In Q. humilis the epicotyl measured 2.5 mm in length and was made up of three or more foliar primordia, a shoot apex, and a short subjacent stem with some trichomes (Figs. 25, 26). Cotyledonary buds had 1-2 pairs of leaf primordia and several trichomes (Figs. 25, 27). Serial transverse sections revealed an incipient connection between the cotyledonary buds and the provascular cylinder (Fig. 28). In Q. coccifera and Q. ilex the epicotyl was 1 mm long and showed two incipient leaf lamina with dorsiventral symmetry. At the base of the epicotyl, the cotyledonary buds were readily visible for the first time (Figs. 29–31). Cotyledonary buds were discernible as a protuberance of undifferentiated and somewhat densely stained cells and originated exogenously as in Q. humilis. In several Q. coccifera and Q. ilex seedlings, buds located in the axils of the cotyledons developed from the outer layers of the cotyledonary tissue instead of embryo axis tissues (Fig. 30). We called these buds adventitious buds in order to differentiate them from cotyledonary buds originated from embryo axis tissue.
Five- to six-week-old seedlings
Within 5–6 wk of germination, the average lengths of the root and shoot for the three oaks were 19 and 5 cm, respectively. The cotyledonary node region measured 1 mm in length in Q. ilex and Q.coccifera and 4 mm in Q. humilis. The cotyledonary region in Q. humilis was readily distinguishable as the two petioles formed a whitish sheath of tissue surrounding the base of the stem (Fig. 32).
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As development proceeded, the cotyledonary region that eventually formed the root–shoot transition region of the seedling became woody showing secondary thickening with a well-developed cambial zone (Figs. 33, 35–37) and, eventually, a phellogen.
Six-month-old seedlings
Seedlings of the three oaks possessed a root between 40 and 70 cm long and a shoot of 15–30 cm. The cotyledonary region of Q. humilis seedlings appeared to be slightly thicker than the shoot and the root.
In each species, the exogenous proliferation of cotyledonary buds produced small clusters of 3–5 buds formed by the first cotyledonary bud and some accessory buds at different stages of development (Figs. 38–42). These clusters were often protected by the cotyledonary petioles (Fig. 41); parenchymatic tissue and/or the periderm of the rapidly expanding seedling axis never engulfed them (Figs. 38–42). Accessory buds had 2–3 pairs of leaf primordia and some hypertrophied scales (Figs. 41–42). The scales were rich in tannins and develop some trichomes. At this stage, the connection of vascular tissue of the buds with the central vascular cylinder was complete (Fig. 40), and it was possible to distinguish some mature xylem and phloem elements in the connection. Secondary vascular connection was established later due to the activity of cambial cells. In each species, the cells of the pith and the xylem parenchymatic cells and the uniseriated rays contained some starch grains.
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DISCUSSION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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). This morphological study confirms the presence of clusters of dormant buds at the cotyledonary node of Q. humilis, Q. ilex, and Q. coccifera seedlings, enabling them to resprout from the stem base. However, no distinct organ of clonal regeneration or lignotuber comparable to those of Q. suber has been observed in these species.
The presence of buds in the axil of the cotyledons at the embryo stage may be related to the number of leaf primordia and its stage of development. The axillary bud originates a bit later than the leaf primordia, and in general, during the second plastochron (Esau, 1985
). Thus, we suggest that in embryos of Q. coccifera and Q.ilex, cotyledonary buds are not formed as leaf primordia at the second node have not yet developed (we consider cotyledons the first pair of leaves); whereas in Q. humilis and Q. suber (Molinas and Verdaguer, 1993a
), cotyledonary buds would be present as more than two pairs of leaf primordia are developed in the shoot apex of mature embryos. In other species, such as Ginkgo biloba (Del Tredici, 1992
), Betula pubescens (Kauppi, Rinne, and Ferm, 1987
), and Eucalyptus cinerea (Graham, Walwork, and Sedgley, 1998
), cotyledonary buds develop after germination, but additional information about the leaf primordia development at the embryo stage of these species is not available.
The anatomy of the cotyledonary node is similar for Q. coccifera, Q. ilex, and Q. humilis, but it differs from that described for Q. suber (Molinas and Verdaguer, 1993a
). In embryos of Q. coccifera, Q. ilex, and Q. humilis, the cotyledonary petioles fuse with the tissue of the embryo axis and only one pair of cotyledonary buds is shown. In these oak species the cotyledonary node does not elongate or elongates only slightly. In contrast, in Q. suber embryos, the cotyledonary petioles are not completely fused to the embryo axis tissue, and in sections passing through the cotyledonary plane, an entire epidermal surface layer extending from the apical dome to the lower insertion point of the cotyledons can be seen (Molinas and Verdaguer, 1993a
). Furthermore, in Q. suber embryos 5–7 pairs of lateral buds develop along the length of the cotyledonary node which, after germination, elongates to 12 cm and remains buried, leaving most of the dormant buds underground.
The presence of buds clusters in the axils of the cotyledons in Q. coccifera, Q. ilex, and Q. humilis seedlings confers the high capacity for regeneration shown by these species when the shoot is cut above the cotyledons, whereas the absence of dormant buds below the soil causes the lack of sprouting when the aerial shoot is eliminated below the cotyledons (Verdaguer et al., 2001
). The present histological analysis shows that dormant buds are strictly limited to the axils of the cotyledons and that no differences in starch accumulation can be observed between the shoot, the upper part of the root, and the rest of the root of these oak species. However, the quantification of starch reserves in above- and underground organs should be undertaken in order to gain a better understanding of starch distribution and its use in response to sprouting.
The proliferation of cotyledonary buds forming clusters of three or more buds in young seedlings is a common characteristic of the Quercus species investigated in this study and in Q. suber (Molinas and Verdaguer, 1993b
). Such accessory buds are exogenous in origin and arise from cells at the base of the cotyledonary buds that are already partially or completely differentiated. However, some buds in Q. coccifera and Q. ilex were determined to arise from cells of the cotyledonary petioles, indicating that this tissue is also capable of generating adventitious buds. The high meristematic potentiality of the cotyledonary node has already been described in resprouting species, such as the burl in Betula pubescens (Kauppi, Rinne, and Ferm, 1987
), the lignotuber in some species of Eucalyptus (Kerr, 1925
; Bamber and Mullette, 1978
; Graham, Walwork, and Sedgley, 1998
) and Banksia (Mibus and Sedgley, 2000
), or the basal chichi in Gingko biloba (Del Tredici, 1992
). Furthermore, cotyledonary nodal explants are commonly used in plant clonal regeneration programs, including woody species (Distabanjong and Geneve, 1996–1997
; Tyagi and Kothari, 1997
; Pradhan, Pattnaik, and Chand, 1998
).
The origin of accessory buds in the species cited above is exogenous, although some buds in Eucalyptus (Chattaway, 1958
) and Banksia (Mibus and Sedgley, 2000
) can also originate endogenously. With the exception of Quercus and Betula pubescens, cotyledonary buds are generally embedded in periderm, and their growth and development are under the surface. Hidden cotyledonary buds should favor survival of the species in comparison to that have non engulfed dormant buds. In Q. coccifera, for example, Malanson and Trabaud (1988)
showed that increasing fire intensity significantly reduced sprout emission, suggesting a negative effect of fire on bud viability. In Mediterranean-type ecosystems, where fire is an important part of the functioning of the system, it would be interesting to analyze the anatomical and morphological characteristics that are involved in the resprouting behavior of key species and determine how this might influence stand structure and composition.
Lignotubers are broadly described as woody organs of storage with a supply of protected buds responsible for sprouting. Lignotubers and burls are found in trees and shrubs of the Mediterranean vegetation including different Quercus species. The term lignotuber was originally applied to the swellings found in the axils of the cotyledons of eucalyptus (Carrodus and Blake, 1970
). These swellings are ontogenetically programmed and not the result of physical or environmental stresses (Carr, Jahnke, and Carr, 1984
; Graham, Walwork, and Sedgley, 1998
). Moreover, in eucalyptus the lignotuber is a transitory organ characteristic of the young forms (Chattaway, 1958
). Ontogenetically programmed lignotubers have also been documented in other species including Banksia (Mibus and Sedgley, 2000
), Cryptocarya alba, Colliguaya odorifera, and Lithraea caustica (Montenegro and Avila, 1982
), and Quercus suber (Molinas and Verdaguer, 1993a, b
). According to James (1984)
, only ontogenetically produced structures rich in carbohydrates and dormant buds located at the stem base should be defined as lignotuber. In mature forms of Q. ilex (Canadell and Rodà, 1991
) and Q. coccifera (Kummerow, Kummerow, and Trabaud, 1990
), thick woody swellings are seldom found at the upper part of the root or root crown. These structures are often referred to as lignotubers because of their high nutrients and carbohydrates content. However, our histological analysis do not reveal any underground structure showing bud and starch accumulation at the seedling stage nor in 2- to 3-yr-old seedlings, as was the case in Q. suber (Molinas and Verdaguer, 1993b
). It is likely that Q. ilex and Q. coccifera seedlings produce new shoots from axillary buds and, as the plant matures, the continuous, secondary growth of the stem base results in the fusion of the main stem and the sprouts. Additionally, axillary buds at the base of sprouts can become buried beneath the developing periderm. This may explain the presence of a thick woody stem base but, to date, no information is available about the accumulation of dormant axillary buds. So, for these structures the term lignotuber should not be used in order to avoid possible misunderstandings.
In 6-mo-old and older seedlings of Q. humilis, the first internode above the cotyledons has a high meristematic capacity as it develops adventitious buds and roots. This characteristic could act in favor of the seedling, allowing it to cope with adverse conditions and possibly leading to the formation of a specialized sprouting structure in mature plants. However, data about the presence in Q. humilis mature trees of swellings, burls, or lignotubers are not available. Further research on older plants is needed to clarify the fate of the cotyledonary buds and the development of the shoot–root transition region in Q. humilis.
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FOOTNOTES |
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4 Author for reprint requests (dolors.verdaguer{at}udg.es
)
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LITERATURE CITED |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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Canadell J. F. Rodà 1991 Root biomass of Quercus ilex in a montane Mediterranean forest. Canadian Journal of Forestry Research 21: 1771-1778[CrossRef]
Carr D. J. R. Jahnke M. G. S. Carr 1984 Initiation, development and anatomy of lignotubers in some species of Eucalyptus. Australian Journal of Botany 32: 415-437[CrossRef]
Carrodus B. B. T. J. Blake 1970 Studies on the lignotubers of Eucalyptus obliqua l'Heri. I. The nature of the lignotuber. New Phytologist 69: 1069-1072[CrossRef][ISI]
Chattaway M. M. 1958 Bud development and lignotuber formation in Eucalypts. Australian Journal of Botany 6: 103-115[CrossRef]
Del Tredici P. 1992 Natural regeneration of Ginkgo biloba from downward growing cotyledonary buds (basal chichi). American Journal of Botany 79: 525-530
Di Pasquale G. G. Garfi 1998 Comparative analysis of the regeneration patterns of Quercus suber and Q. pubescens after grazing exclusion in the forest. Ecologia-Mediterranea 24: 15-25
Distabanjong K. R. L. Geneve 1996–1997 Multiple shoot formation from cotyledonary node segments of eastern redbud. Plant, Cell, Tissue, and Organ Culture 47: 247-254
Ducrey M. M. Turrel 1992 Influence of cutting methods and dates on stump sprouting in holm oak (Quercus ilex L.) coppice. Annals Science Forestry 49: 449-464
Esau K. 1985 Anatomia vegetal, 3rd ed. Omega Edition, Barcelona, Spain
Fuente de la J. J. Sardans F. Rodà 1997 Creixement de rebrots d'alzina en una brolla postincendi en el període entre tres i sis anys després de fertilitzar-los. Orsis 12: 117-125
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James S. 1984 Lignotubers and burls: their structure function and ecological significance in mediterranean ecosystems. Botanical Review 50: 225-266
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Montenegro G. G. Avila 1982 Presence and development of lignotubers in shrubs of the Chilean matorral. Canadian Journal of Botany 61: 1804-1808
Pausas J. G. 1997 Resprouting of Quercus suber in NE Spain after fire. Journal of Vegetation Science 8: 703-706
Pradhan C. S. Pattnaik P. K. Chand 1998 Rapid in vitro propagation of East Indian Rosewood (Dalbergia latifolia Roxb.) through high frequency shoot proliferation from cotyledonary nodes. Journal of Plant Biochemistry and Biotechnology 7: 61-64[ISI]
Retana J. J. M. Espelta M. Gracia M. Riba 1999 Seedling recruitment. In F. Rodà, J. Retana, C. A. Gracia and J. Bellot [eds.], Ecology of Mediterranean evergreen oak forests, Ecological studies, vol. 137, 89–103. Springer-Verlag, Berlin, Germany
Retana J. M. Riba C. Castell J. M. Espelta 1992 Regeneration by sprouting of holm-oak (Quercus ilex) stands exploited by selection thinning. Vegetatio 99–100: 355-364
Trabaud L. 1987 Natural and prescribed fire: survival strategies of plants and equilibrium in mediterranean ecosystems. In J. D. Tenhunen [ed.], Plant response to stress, vol. 15, 607–621. Springer-Verlag, Berlin, Germany
Tyagi P. S. L. Kothari 1997 Micropopagation of Capparis decidua through in vitro shoot proliferation on nodal explants of mature tree and seedling explants. Journal of Plant Biochemistry and Biotechnology 6: 19-23
Verdaguer D. E. García-Berthou G. Pascual P. Puigderrajols 2001 Sprouting of seedlings of three Quercus species (Q. humilis Miller, Q. ilex L., and Q. suber L.) in relation to repeated pruning and the cotyledonary node. Australian Journal of Botany 49: 67-74[CrossRef][ISI]
Verdaguer D. M. Molinas 1997 Development and ultrastructure of the endodermis in the primary root of cork oak (Quercus suber). Canadian Journal of Botany 75: 769-780
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