In this article we will discuss about Taxales. After reading this article you will learn about: 1. Taxonomic Position of Taxales 2. General Characters of Taxales 3. Types.
Taxonomic Position of Taxales:
Family Taxaceae, along with 5 of its living genera and 200 species, was usually treated as belonging to Coniferales until the beginning of the twentieth century. Besides the present members of Taxales, certain members of Podocarpaceae and Cephalotaxaceae, such as Podocarpus and Cephalotaxus, were also included under Taxaceae.
It was Professor Birbal Sahni (1920), a well-known Indian botanist, who suggested that Cephalotaxus, Taxus and Torreya should be treated under Taxales. Florin (1948, 1951) has, however, opined that Cephalotaxus should not be included under Taxales but under Coniferales because it is a true Conifer.
According to Florin (1948) Taxales include six genera, namely Taxus, Austrotaxus, Pseudotaxus (=Nothotaxus), Torreya, Amentotaxus and Palaeotaxus. Palaeotaxus is the only fossil genus of the family. Sporne (1965) has also followed the same classification.
According to Turrill (1959), out of the total 19 species of Taxales, 7 species belong to Taxus, 1 each to Austrotaxus and Nothotaxus, 6 species to Torreya and 4 to Amentotaxus. Ramanujam (1976) mentioned that the order Taxales is poorly represented in the fossil flora of India.
General Characters of Taxales:
1. The sporophytic plant body consists of evergreen, slow-growing, profusely branched shrubs or small trees.
2. The leaves are simple, linear with acute apex. They are spirally arranged on the branches.
3. The secondary wood is compact and pycnoxylic.
4. The wood is elastic because tertiary spirals are present on the walls of the tracheids.
5. The resin canals are absent in wood or leaves.
6. Plants are unisexual with only a few exceptions.
7. Micro-sporangiophores are arranged in the form of small cones.
8. On each micro-sporangiophore 2-8 pollen sacs are present on a scale-like or peltate disc.
9. Prothallial cells are absent in the male gametophyte.
10. The ovule is solitary and borne terminally on a dwarf shoot.
11. An aril is present at the base of each ovule.
12. Pollination takes place by pollination drop mechanism.
13. The embryo is dicotyledonous.
14. The seeds are endospermic.
Types of Taxales:
(i) Palaeotaxus:
It is the only known fossil representative of Taxales. Its fertile specimens have been discovered from the rocks of the Triassic period of Mesozoic era. Plants had solitary terminal ovules, a characteristic feature only of Taxales.
The ovule and later on the seed was borne at the tip of a short shoot. Its lower parts were enclosed by spirally arranged bracts (Fig. 12.22 A). Aril was also present. Except at the top of the micropylar end, the ovule was enclosed by the aril.
(ii) Austrotaxus:
It is a living Taxad, represented by only one species. Plants are found growing in New Caledonia. Its male cones are quite peculiar and made up of micro-sporangiophores.
Each micro-sporangiophore remains subtended by a bract, and only because of this peculiarity Nakai (1938) suggested that Austrotaxus should be placed in a separate family Austrotaxaceae. Each micro-sporangiophore terminates into a synangium formed by the fusion of 3 or 4 microsporangia.
(iii) Pseudotaxus (= Nothotaxus):
It is found growing these days in a small region of the forests of east China. It is also represented by only one species. Its male cones are also made of micro-sporangiophores similar to Austrotaxus, but here the bracts are irregularly distributed.
Each micro-sporangiophore of Pseudotaxus is actually a highly reduced male cone, and, therefore, the entire structure, formed by a group of micro-sporangiophores, is a compound structure.
(iv) Torreya:
It is represented by 5 species and occurs in some east-Asian countries and some parts of USA, including Florida and California. Plants are mostly large trees attaining a height of 25 to 30 metres but some are only shrubs. The leaves of some of the species (e.g. Torreya californica) attain a length of 6 to 9 cm. or even more.
Only a single vascular bundle is present in each leaf. Wood parenchyma is present in the secondary wood of the stem. The tertiary spirals are well-developed in the tracheids. In the ovule (Fig. 12.22B) the aril remains fused with the integument for most of its length except near the micropyle.
There is no vascular bundle in the integument of its own. Two vascular bundles, however, run inside the aril and reach up to the apex of the mature ovule.
From each of these vascular bundles develops a branch which passes through a foramen and supplies the stony layer of the ovule. Each of these branches then divides and forms a loop which surrounds the mature ovule or seed. The female gametophyte in the mature seed develops very fast and without any definite division pattern.
The ultimate result is the formation of an irregular- shaped surrounding tissue. The micro-sporangiophores (Fig. 12.22C) are not peltate. They are dorsiventral, scale-like structures. Each micro-sporangiophore contains four pendulous microsporangia or pollen sacs.
Male prothallial cells are absent. The size of the male gamete cells is quite unequal. Each female gametophyte contains only one archegonium. The ventral canal nucleus is absent in the archegonium of Torreya.
(v) Amentotaxus:
This occurs in several parts of some east-Asian countries and represented by only four species. Its chief characteristic feature is shown by its seed which remains surrounded by its aril and several bracts, and all these structures develop at the terminal part of a peduncle. The peduncle attains a length of 1 – 2 cm. The aril remains fused with the integument, and there is no vascular supply at all to the integument.