Taxon Details: Megalastrum canacae (Holttum) Holttum
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Combination for Megalastrum canacae (Holttum) Holttum
Combination for Megalastrum canacae (Holttum) HolttumFamily:
Dryopteridaceae (Pteridophyta)
Dryopteridaceae (Pteridophyta)
Scientific Name:
Megalastrum canacae (Holttum) Holttum
Megalastrum canacae (Holttum) Holttum
Primary Citation:
Gard. Bull. Singapore 39(2): 161. 1986
Gard. Bull. Singapore 39(2): 161. 1986
Accepted Name:
This name is currently accepted.
This name is currently accepted.
Description:
by: G. Rouhan & R.C. Moran
Type: Mauritius. Canaca, [20°24'11''S, 57°30'42''E], 1868, Barkly s.n. (holotype, K!)
Description: Rhizomes erect, scaly, the scales up to 35 mm long; fronds up to 2.5 m long; scales of the petiole bases 10-15 × 1.0-1.5(-2.0) mm, linear, subentire, dark brown, spreading to ascending; rachises with similar scales but often much less densely; laminae up to 1.2 m long, bipinnate-pinnatisect at base, bipinnate-pinnatifid medially; basal pinnae 25-50 cm long, the stalk up to 2 cm long, inequilateral, the pinnules on the acroscopic side slightly reduced toward the base of the pinna, the two or three first basiscopic pinnules with lobes pinnatifid and obtuse to rounded apices; pinna rachises abaxially puberulent and more or less densely glandular, the hairs erect, 0.2-0.5 mm long, 3-5-celled, the glands spherical, sessile, ca. 0.1 mm diam., 1-celled, scaly, the scales 4-7(-10) × 0.3-0.5(-1.0) mm long, linear, entire, flat (non-bullate), dark brown, adaxially densely pubescent, the hairs spreading, 0.8-1.2 mm long, 5-7-celled, septate, non-glandular; costules abaxially puberulent, the hairs spreading to somewhat antrorse, 0.1-0.4 mm long, 1-3-celled, glands as those on pinna rachises but fewer, scaly, the scales 1.5-3.5 × 0.2-0.4 mm, flat, entire, brown, adaxially pubescent, the hairs antrorse, 0.6-1.0 mm long, 5-7-celled, not glandular, septate; laminar tissue between veins glabrous on both surfaces, abaxially sparsely glandular, the glands spherical, sessile; veins to 8 pairs per pinnules lobe, visible on both surfaces, ending behind lamina margins in enlarged clavate tips (conspicuous hydathodes), abaxially sparsely glandular and puberulent, the hairs 0.1-0.4 mm long, 2- or 3-celled, adaxially eglandular, puberulent to glabrous, the hairs ca. 0.3-0.5 mm long, 3-5-celled; laminar margins sparsely ciliate, the hairs ca. 0.3-0.5 mm long, 2- or 3-celled; indusia present, persistent (rarely deciduous), circular, 0.3-0.5 mm, pubescent, the hairs 0.1-0.3 mm long, 1- or 2-celled, also present on the margins.
Etymology: The specific epithet canacae refers to the crater Kanaka (or Canaca) in Mauritius, where the type specimen was collected.
Distribution: Terrestrial, in shady stations of wet mesotherm forests, in ravines, from 1200 m. Rare in Mauritius and La Réunion. It is very rare or it even might be now extinct in Mauritius where it was last collected in 1985 by Strahm, who wrote on the herbarium label 'only a few individuals, rare'. The only other known locality in Mauritius is Trou Kanaka, where it has not been collected since 1868, although it was searched for at this locality in the early 1980s (Holttum, 1983).
Comments: Megalastrum canacae is the only species of the genus in Paleotropics with glabrous laminar tissue between veins on both faces. Other useful characters for distinguishing this species are the numerous dark brown scales extending from the petiole to the rachis, the antrorse hairs on the adaxial surfaces of the costules and veins, and persistent indusia. Contrary to Holttum's statement (1983), the basal basiscopic pinnules of basal pinnae are not necessarily significantly shorter than those next to them. Thus, examining more specimens, especially recent collections from La Réunion, this character appears as being not as valuable for distinguishing the species. The density of spherical glands observed on the abaxial surfaces can vary between specimens from almost none (Grangaud 3, 4, and 5) to many (Barkly s.n., holotype).With few exceptions where indusia are extremely reduced and present on only a few pinnules (Grangaud 5 and 1243bis), indusia are conspicuous, even when dried and shriveled on dry specimens. The following name should refer to M. canacae according to some restrictions, here mentioned: Nephrodium oppositum (Kaulf. ex Spreng.) Hook., Spec. Fil. 4: 136, fig. 268. 1862, based on Sieber 36 (from Mauritius) non Aspidium oppositum Kaulf. ex Spreng., Syst. Veg. 4 (ed. 16): 108. 1827, nec Lastrea opposita (Kaulf. ex Spreng.) C. Presl., Tent. Pterid. 77. 1836, based on collector unknown s.n. from South Africa. The type specimen of Aspidium oppositum Kaulf. ex Spreng., has never been located. The protologue states that it was collected from the Cape of Good Hope in South Africa. Hooker had to base his description of Nephrodium oppositum (based on A. oppositum) on Sieber 36, from Mauritius, not South Africa. Morton confirmed that Hooker based his description on a Mauritian specimen, stating on his photos of Sieber 36 (MO!, NY! ex H) that the 'C.B.S.'(Caput Bonae Spei) in the original description should be considered as a mistake, and that this species is found only in the Mascarene Islands. Because there is no reason to prefer this hypothesis, we followed Holttum (1983) who argued that the description of A. oppositum is too brief and therefore inadequate for identification. Thus, it is not possible to conclude that the species described as Aspidium oppositum Kaulf. ex Spreng. and typified as from South Africa was different from M. canacae growing in the Mascarene Islands. Holttum (1983) gave detailed explanations on the two illegitimate names Dryopteris mascarenarum Urb. (Symb. Antill. 4: 14. 1903, nom. illeg. superfl., pro parte as to the specimen Sieber 36 only.) and Ctenitis mascarenarum (Urb.) Tardieu (Notul. Syst. 15: 92. 1955, nom. illeg., superflu. pro parte as to the specimen Sieber 36 only) that also refers to M. canacae
by: G. Rouhan & R.C. Moran
Type: Mauritius. Canaca, [20°24'11''S, 57°30'42''E], 1868, Barkly s.n. (holotype, K!)
Description: Rhizomes erect, scaly, the scales up to 35 mm long; fronds up to 2.5 m long; scales of the petiole bases 10-15 × 1.0-1.5(-2.0) mm, linear, subentire, dark brown, spreading to ascending; rachises with similar scales but often much less densely; laminae up to 1.2 m long, bipinnate-pinnatisect at base, bipinnate-pinnatifid medially; basal pinnae 25-50 cm long, the stalk up to 2 cm long, inequilateral, the pinnules on the acroscopic side slightly reduced toward the base of the pinna, the two or three first basiscopic pinnules with lobes pinnatifid and obtuse to rounded apices; pinna rachises abaxially puberulent and more or less densely glandular, the hairs erect, 0.2-0.5 mm long, 3-5-celled, the glands spherical, sessile, ca. 0.1 mm diam., 1-celled, scaly, the scales 4-7(-10) × 0.3-0.5(-1.0) mm long, linear, entire, flat (non-bullate), dark brown, adaxially densely pubescent, the hairs spreading, 0.8-1.2 mm long, 5-7-celled, septate, non-glandular; costules abaxially puberulent, the hairs spreading to somewhat antrorse, 0.1-0.4 mm long, 1-3-celled, glands as those on pinna rachises but fewer, scaly, the scales 1.5-3.5 × 0.2-0.4 mm, flat, entire, brown, adaxially pubescent, the hairs antrorse, 0.6-1.0 mm long, 5-7-celled, not glandular, septate; laminar tissue between veins glabrous on both surfaces, abaxially sparsely glandular, the glands spherical, sessile; veins to 8 pairs per pinnules lobe, visible on both surfaces, ending behind lamina margins in enlarged clavate tips (conspicuous hydathodes), abaxially sparsely glandular and puberulent, the hairs 0.1-0.4 mm long, 2- or 3-celled, adaxially eglandular, puberulent to glabrous, the hairs ca. 0.3-0.5 mm long, 3-5-celled; laminar margins sparsely ciliate, the hairs ca. 0.3-0.5 mm long, 2- or 3-celled; indusia present, persistent (rarely deciduous), circular, 0.3-0.5 mm, pubescent, the hairs 0.1-0.3 mm long, 1- or 2-celled, also present on the margins.
Etymology: The specific epithet canacae refers to the crater Kanaka (or Canaca) in Mauritius, where the type specimen was collected.
Distribution: Terrestrial, in shady stations of wet mesotherm forests, in ravines, from 1200 m. Rare in Mauritius and La Réunion. It is very rare or it even might be now extinct in Mauritius where it was last collected in 1985 by Strahm, who wrote on the herbarium label 'only a few individuals, rare'. The only other known locality in Mauritius is Trou Kanaka, where it has not been collected since 1868, although it was searched for at this locality in the early 1980s (Holttum, 1983).
Comments: Megalastrum canacae is the only species of the genus in Paleotropics with glabrous laminar tissue between veins on both faces. Other useful characters for distinguishing this species are the numerous dark brown scales extending from the petiole to the rachis, the antrorse hairs on the adaxial surfaces of the costules and veins, and persistent indusia. Contrary to Holttum's statement (1983), the basal basiscopic pinnules of basal pinnae are not necessarily significantly shorter than those next to them. Thus, examining more specimens, especially recent collections from La Réunion, this character appears as being not as valuable for distinguishing the species. The density of spherical glands observed on the abaxial surfaces can vary between specimens from almost none (Grangaud 3, 4, and 5) to many (Barkly s.n., holotype).With few exceptions where indusia are extremely reduced and present on only a few pinnules (Grangaud 5 and 1243bis), indusia are conspicuous, even when dried and shriveled on dry specimens. The following name should refer to M. canacae according to some restrictions, here mentioned: Nephrodium oppositum (Kaulf. ex Spreng.) Hook., Spec. Fil. 4: 136, fig. 268. 1862, based on Sieber 36 (from Mauritius) non Aspidium oppositum Kaulf. ex Spreng., Syst. Veg. 4 (ed. 16): 108. 1827, nec Lastrea opposita (Kaulf. ex Spreng.) C. Presl., Tent. Pterid. 77. 1836, based on collector unknown s.n. from South Africa. The type specimen of Aspidium oppositum Kaulf. ex Spreng., has never been located. The protologue states that it was collected from the Cape of Good Hope in South Africa. Hooker had to base his description of Nephrodium oppositum (based on A. oppositum) on Sieber 36, from Mauritius, not South Africa. Morton confirmed that Hooker based his description on a Mauritian specimen, stating on his photos of Sieber 36 (MO!, NY! ex H) that the 'C.B.S.'(Caput Bonae Spei) in the original description should be considered as a mistake, and that this species is found only in the Mascarene Islands. Because there is no reason to prefer this hypothesis, we followed Holttum (1983) who argued that the description of A. oppositum is too brief and therefore inadequate for identification. Thus, it is not possible to conclude that the species described as Aspidium oppositum Kaulf. ex Spreng. and typified as from South Africa was different from M. canacae growing in the Mascarene Islands. Holttum (1983) gave detailed explanations on the two illegitimate names Dryopteris mascarenarum Urb. (Symb. Antill. 4: 14. 1903, nom. illeg. superfl., pro parte as to the specimen Sieber 36 only.) and Ctenitis mascarenarum (Urb.) Tardieu (Notul. Syst. 15: 92. 1955, nom. illeg., superflu. pro parte as to the specimen Sieber 36 only) that also refers to M. canacae