sign in sign up
<<
Home , Austrotaxus, Microcachrys, Phyllocladus, Pseudotaxus, Saxegothaea, Taxaceae

OPINION Inclusion of in a separate order,

D. D. Pant

Taxus and its related genera, viz. , are common to the and their new times called taxinean spirals, but these , and order Taxales. The present article is occur in as well. were unquestionably included in the therefore intended to have a fresh look at Among characters of which have Pinales (= Coniferales) although they the similarities and differences between been mentioned as altogether different were usually included in a family of their Taxaceae and other to enable us from those of all other Pinales, are its radi- own, the Taxaceae, inclusive of Cephalo- to decide whether we can continue to ally organized peltate microsporophylls taxus by Coulter & Chamberlain1 or ex- keep the Taxaceae as a family within the with sporangia attached on the adaxial, clusive of Cephalotaxus by Pilger2, Pinales or to include them in that family inner side. However, other genera of the who placed Cephalotaxus in a separate under a separate order, the Taxales. Taxaceae have dorsiventral microsporo- family, the . However, As Chamberlain6 had pointed out, ‘the phylls with microsporangia attached on in 1920, Sahni3 suggested that Taxus, grouping into families and sequence of the abaxial, underside like those of the Torreya and other closely-related genera families will depend upon each investi- Pinales. Thus, if we take the character of and Cephalotaxus were so different from gator. If he is an anatomist, anatomy will peltate microsporophylls into considera- other conifers and they should be inclu- determine the treatment. If strobili are tion we can widely separate Taxus alone ded in a separate order Taxales. Sahni’s considered more important, they will into Taxales but the microsporophylls of suggestion about a wider separation of determine the grouping and sequence. If the other genera conform with those of Taxus in a separate order Taxales was gametophytes are emphasized, there will the Pinales. supported by Florin4, but he pointed out be still another arrangement’. Therefore, The development and structure of the that Cephalotaxus should continue un- in order to eliminate individual bias it male and the female gametophytes in the disturbed in the Pinales since it did have seems to be necessary to take diverse Taxaceae are quite comparable with those a female cone, although it was much characters of the Pinales, exclusive of in the rest of the Pinales. Finally an intri- reduced but Taxus alone was radically Taxaceae and of the Taxaceae into con- cate character of the Pinales like the different from Coniferales because it had sideration in order to decide whether the tiered arrangement of the proembryo, no female cone and its female dwarf Taxaceae need to be separated widely which is characteristic of all pro- shoot ended in a terminal ovule (Figures under Taxales as suggested by Sahni3 embryos is also found in the Taxaceae as 1 a, b and 2 a). The unique peltate radi- and Florin4 or kept within the Pinales shown by Sterling7 (see Figure 3). ally organized microsprophylls of Taxus (= Coniferales). The basic chromosome number too is (Figure 1 c, d) seemingly supported the In their habitats and habit the Taxaceae n = 12 in Taxus, but Torreya and Amen- above wider separation of the Taxales. are quite like many other Pinales. They totaxus have n = 11 (see Khoshoo8 and Another point which may have prompted are woody pycnoxylic and Mehra9) and these numbers too are quite in Florin4 to plead for a wider separation of inhabiting locations like those of the conformity with those prevalent in most of Taxaceae in the Taxales was the lack of Pinales. Their leafy shoots and simple the Pinales. any fossils which showed characters are similar to those of many coni- linking Taxaceae with any other conifers. fers. Their secondary xylem no doubt is a The earliest taxads like Palaeotaxus peculiar in showing tertiary spirals, some- b redeviva Florin (Figure 2 b) and Marskea thomasii Harris (Figure 2 c) take us back into the Lower and Middle , res- pectively, but there they leave us in the d lurch about their links with any other groups of conifers or other . The only of Taxus-like fossil shoots c reported from the Palaeozoic Lower Gondwana rocks is Searsolia oppo- sitifolia Pant & Bhatnagar5 but except for some vague carbonless rounded impres- sions of frutification-like bodies on a shoot, its reproductive parts are unknown a b c d Figure 2. a, longisec- (Figure 2 d). Thus except for giving tion of female dwarf shoot and terminal overwhelming weight to the evidence of Figure 1. . a, Twig-bearing ovule. b, c, Female flowers of Palaeo- a terminal ovule and the absence of a mature with a surrounding ; b, taxus rediviva and Marskea thomasii 3 Longitudinally split mature seeds inside aril; respectively; d, Searsolia oppositifolia, female cone in the Taxales, neither Sahni c, Twig showing mature male ‘flowers’; showing a leafy shoot with secondary 4 nor Florin seems to have taken into con- d, A peltate microsporophyll. a, b, after foliate branches and three shortly stalked sideration the various characters which Kerner, 1895; c, d, after Kerner, 1913. globose over the main axis.

278 CURRENT SCIENCE, VOL. 79, NO. 3, 10 AUGUST 2000 OPINION

d a b c a c

a b c′ d′ d e f g Figure 4. Hypothetical longisections of different onyogenetic stages of apices Figure 5. Buriadia heterophylla. a, Bran- of an early Taxus ancestor. a, Youngest ched shoot showing attached ovules; stage with raised apical meristem and b, Portion of shoot showing an attached shoulders which will form scales; b, as a anatropous ovule among leaves; c, A but older with apex flattening, c, as b but reconstructed hypothetical shoot showing older, two ovules marked ‘o’ are beginning laterally attached solitary seeds among to form; c′, as c but with one median leaves on branches of all orders; d, Axis ovule. d′, as c but older, the dormant of a (hypothetical) with seeds as in shoot apex bulges between the two integu- Buriadia but borne only on some unre- mented ovules; d′ as c′ but older with a duced lateral branches; e, Axis of a plant terminal ovule (all after Harris10). (hypothetical) with reduced lateral bran- ches; f, Hypothetical axis like e but with b c fertile short shoots clustered around a be fully justified but their wider separa- specialized lateral branch; g, Reconstruc- Figure 3. Taxus. a, Secondary pro- ted shoot of Lebachia piniformis with ter- embryo with upper (U), suspensor (S) and tion in a separate order of their own, the minal cones. Figure on the right side embryonal (E) tiers; b, Same at a later Taxales, is supported solely by the termi- shows a cone with its basal part longitudi- stage showing a few surrounding gameto- nal position of their ovules in their nally split (after Pant and Nautiyal13). phytic cells; c, Young proembryo (e) with female dwarf shoots and according to differentiated embryonal tubes and with one 12 of its cells separated (all after Sterling7). Miller (as mentioned above) this termi- nal position too is shared by a few mem- 6. Chamberlain, C. J., Gymnosperms, Struc- bers of the Pinales. Moreover, the ovules ture and Evolution, University of Chi- could have become terminal by the abor- cago Press, Illinois, 1935. 7. Sterling, C., Bull. Torrey Bot. Club, Therefore, taking into consideration all tion of their dwarf shoot apices as sug- 10,11 1948, 75, 147–165. the above characteristics of the Taxaceae gested by Harris . Further, the cones 8. Khoshoo, T. N., in Proc. Summer School we find that the only character which is of Pinales could indeed be derived from in Botany, Darjeeling (eds Maheshwari regarded as categorically supporting their the scattered condition of female dwarf P., Johri, B. M. and Vasil, I. K.), Min. wider separation into Taxales are their shoots like those in Taxus to a condition Sci. Res. and Cul. Affairs, New Delhi, terminal ovules in their female dwarf where they become crowded and inserted 1962, pp. 119–135. shoots. However, as pointed out by at each node in pinalean cones as envis- 9. Mehra, P. N., Conifers, Gnetophytes and 13 14 Phylogeny of Gymnosperms, 1988. Harris10,11, a lateral ovule in a fertile aged by Pant & Nautiyal and Pant (see Figure 5). The cumulative evidence 10. Harris, T. M., Rev. Palaeobot. Palynol., dwarf shoot like that of Lebachia, can be 1976, 21, 119–134. of all the diverse characters of Taxus and assumed to have become terminal by the 11. Harris, T. M., The Yorkshire Jurassic abortion of the dwarf shoot apex and its allied genera, as discussed above, is Flora 5, Coniferales British Museum Harris has cited examples from the angio- therefore, adjudged as being strongly (Nat. Hist.) London, 1979. sperms where this actually occurs (Figure against their wider separation in the order 12. Miller Jr., C. N., in Origin and Evolution 4). Moreover, as mentioned by Miller12 Taxales. of Gymnosperms (ed. Beck, C. B.), in his table 10.2 dealing with character Columbia University Press, NY, 1988. states of the OTU used in the CLINCH 13. Pant, D. D. and Nautiyal, D. D., Philos. Trans. R. Soc. London B., 1967, 774, 27– and PAUP analysis, terminal ovules occur 1. Coulter, J. M. and Chamberlain, C. J., 48. also in Phyllocladus, Microcachrys Morphology of Gymnosperms, University of Chicago Press, Illinois, 1917. 14. Pant, D. D., J. Indian Bot. Soc., 1977, Saxegothaea of the , 56, 23–37. Araucaria of the and Cal- 2. Pilger, R., in Die Naturlichen Pflanzen- familien (eds Engler, A. and Prantl, K.), litris of the . Ed. 2 Bd. 13 Engelmaun, Leipzig, 1926. We may thus conclude after taking all 3. Sahni, B., Ann. Bot., 1920, 34, 117–133. the above facts into consideration, that 4. Florin, R., Bot. Gaz., 1948, 110, 31–39. the assignment of Taxus and its allied 5. Pant, D. D. and Bhatnagar, S., Palaeonto- D. D. Pant lives at 106, Tagore Town, genera to the family Taxaceae appears to graphica B, 1975, 152, 191–199. Allahabad 211 002, India.

CURRENT SCIENCE, VOL. 79, NO. 3, 10 AUGUST 2000 279