Notes on Taxaceae | Plants

The below mentioned article provides a note on taxaceae.

The family Taxaceae comprise of evergreen trees with flat, linear, apparently distichous leaves and without resin canals. They are mostly confined to the Northern Hemisphere. Male flowers solitary or in small, axillary cones. Pollen grains wing­less and do not cut off any prothallial cell during male gametophyte formation.

Micro- sporophylls peltate or apically thickened, with 3-9 microsporangia. Ovules not in strobili, axillary, terminal on dwarf shoots, seated on a meristematic tissue whose megasporophyll nature is not clear. Seed erect, hard, covered by a fleshy, coloured aril which is the only part in the tree free from the alkaloid taxin.

There are 5 genera with about 15 species:

Tribe Torreycae:

Genera: 1. Amentotaxus (1 sp.): China. Known as Tertiary fossil.

2. Torreyea (5 spp.): China, Japan, California, Florida. Known as fossil since Upper Jurassic.

Tribe Taxeae:

Genera: 3. Austrotaxus (1 sp.): New Caledonia.

4. Nothotaxus (1 sp.): China.

5. Taxus (Yew- 7 to 8 spp.): England, Continental Europe, Asia Minor, Himalaya, China, Japan, Philippines, Celebes, N. America, Mexico. Known as fossil since Jurassic.

Fossils named Palaeotaxus are known from Upper Triassic, as also Taxoxylon wood from Cretaceous and Taxites twigs from Jurassic to recent. Taxites is known from Jurassic Indian Upper Gondwana in the Rajmahal series.

Indian Species of Taxaceae:

1. Taxus baccara L.:

Spreading from England and Europe over to temperate Himalaya, Khasia Hills, Naga Hills, Manipur and Upper Burma. Up to 50 ft. tall and often very old. Hard timber. Used as fodder in spite of the alka­loid (Fig. 735). (Distribution shown on map in Figure 731).

T. wallichiana Zucc. with narrower leaves has sometimes been considered as a separate species but it is doubtful whether it is different from T. baccata.

Genus Taxus:

The Sporophyte Plant:

Taxus is an evergreen, densely branched tree of medium height which is known to be long living. Some of the British yew trees are claimed to be 2000 years old. About 7 or 8 species are distributed all over the Northern Hemisphere. Taxus baccata L. is the predominant species spreading from England to Japan over the Himalaya, to the Pacific Islands and to North America going as south as Mexico.

The ‘yew’ tree is of historical importance in Europe. The trunk of the tree is quite thick and is usually formed by the fusion of several basal branches.

The leaves are dark-green, 2 to 4 cm long, more or less broadly linear (Fig. 735 A & B) with a short stalk-like portion broadening in a de-current base and ending in a spinous tip. There is a single median vein. They are spiral in arrangement but in the shoots they are twisted to give a distichous appearance.

Although persistent, they fall off after some time leaving the leaf base as permanent ridges. All the leaves are of one type and the shoots also are uniform. The poisonous alkaloid taxin is present in the leaves and all parts of the plant excepting the aril. When present in large quan­tity it may be injurious to cattle. Scaly winter buds develop on shoots for overwintering.

The anatomy of the leaf and the stem generally resembles Pinus with certain differences. In the leaf the mesophyll is differentiated into palisade and spongy parenchymas and there is a single vascular bundle. Otherwise the leaf presents the same xerophytic structure.

The stem anatomy differs in having no resin duct, in the medul­lary rays being simpler and always uniseriate, and in the cortex and the pith occupying a lesser area of the stem.

The tracheides have uniseriate bordered pits and they show a tertiary spiral thickering (Fig. 735G) rendering the wood more elastic. This was the reason why yew was the favourite of construction of bows among the European medie­val archers. The root structure is similar to that of Pinus.

Taxus is dioecious although sometimes male and female shoots are found on the same tree. The flowering shoots are borne on the underside of the shoots of the pre­vious year at the axils of leaves (Fig. 735A & B). Flowering in India takes place in early spring (February-March).

The male strobilus or flower (Fig. 735C) is a solitary, stalked, sub-globose, short catkin of 6 to 10 microsporophyll’s borne on an axis. There are a few scaly bracts at its base. Each microsporophyll (Fig. 735D) is peltate (as in Equisetum) with 5 to 9 microsporangia hanging below the peltate disc.

Each microsporangium has a wall, a tapetum inside, and contains numerous microspores or pollens which are not winged. The microsporangia split and curl out when ripe to liberate the pollens.

The female shoot (Fig. 735E & F) arises in the axil of a leaf on the previous year’s branch as a small bud. It bears a number of closely overlapping scale leaves and a growing apex on the axis which soon ceases to function after the development of a secondary lateral shoot in the axil of one of its top scales.

This secondary shoot may be called the female flower as it bears a few scale leaves and ends in a single terminal ovule. The structure of the ovule is as in Pinus excepting the aril which is described below.

Gametophytes, Pollination, Fertilisation:

Pollination is anemophilous as in Pinus but the wingless pollens are shed in the uninucleate stage. The development of the female gametophyte also resembles Pinus excepting a few minor differences. The archesporial cells seem to be hypodermal in origin but the megaspore mother cells are found deep within the nucellus.

In Taxus cases are known where different megaspore mother cells have given rise to more than one female gametophyte. But in such cases only one gametophyte survives to the last.

The megaspore mother cell gives rise to a linear tetrad of megaspores of which only the lowest one is functional. But, here again, cases are known in Taxus where more than one megaspore from the same linear tetrad start forming female gametophytes though only one of them persists to the end.

As in Pinus, the female gametophyte develops several archegonia. An important point of difference is that no cell wall develops bet­ween the ventral canal nucleus and the egg nucleus before the former disintegrates so that there is no ventral canal ‘cell’ in Taxus.

There is a big differences in the development of the male gametophyte between pinus and taxus. The wingless pollen of Taxus (fig . 736 A), shed in the uninucleate stage, begins germination after it is lodged on the tip of the mega-sporangium nucellus. The first division of its nucleus results in a generative cell and a tube cell (Fig. 736B) so that the prothallial cells are completely eliminated.

Thereafter it develops (Fig. 736 G & D) as in Pinus. The two mule gametes are always of unequal size. The pollen tube reaches the female gametophyte top piercing through the nucellus very quickly, usually two months before the archegonia are formed, but the actual bursting of the pollen tube and fusion of the male gamete and the egg take place later. Fertilisation results in the zygote or the oospore.

Sporophyte Embryo, Seed and Germination of seed:

Development of the embryo is similar to Pinus but the proembryo fills up the whole of the zygote cavity and its tiers are irregular. Usually there is a single embryo from a proembryo there being no cleavage polyembryony. But, several archegonia within the same gametophyte may send down embryos restating in simple polyembryony.

How­ever, as in Pinus, a single embryo in a seed is the usual rule. The embryo shows two cotyledons, a plumule and a radicle which are surrounded by the endosperm (gametophytic tissue) in the young stage (Fig. 736E).

During the development of the seed (Fig. 736E) the integument, three-layered as in Pinus at the beginning, dies up into a hard testa. A peculiarity is the development of the aril. The aril is present in the ovule as a low collar surrounding its base (Fig. 735F). As the seed matures the aril grows as a fleshy envelope covering the seed ex­cept at the top (Figs. 735A & 736E) giving the seed a berry-like appearance.

The aril, at first green, becomes red in the mature seed. It is edible, much liked by birds and is the only part of the Taxus plant devoid of the poisonous alkaloid Taxin. The seed itself, however, is poisonous. The morphological nature of the aril has been greatly discussed.

It has been supposed by different authors to be a modified ovuli­ferous scale, an epimatium (as in Podocarpus), a delayed outer layer of the integument, a second integument, a cupule-like structure, a scale, etc. In the other genus Torreya the aril completely covers the seed.

The germination of the seed is hypogeal with the first two linear cotyledonary leaves piercing the soil. The primary root persists.

Economic Use of Taxus:

Taxus wood is very durable but is not much used in India because of its relative inaccessibility. It is locally used for poles, ploughs, cart axles, etc. Formerly it used to be of great demand for bows. As the poisonous alkaloid content is usually low it is often used as fodder. In Ladakh the bark is used as a sort of tea. The aril is eaten by birds.

Affinities of the Coniferales and The Taxales:

That the Coniferales evolved out of the Cordaitales is now accepted by most authori­ties. Florin (1951) has shown by the comparison of fossils that the female strobilus of the Pinaceae has evolved from the Cordaitales through Lebachiaceae and Voltziaceae.

The reduction of the broader leaves of the Cordaitales to the needle leaves of Pinaceae has also been taken for granted although fossil evidences are not quite so strong.

Within the Coniferales the two families Pinaceae (Abietaceae) and Araucariaceae both seem to be very primitive, there ancestry being traceable perhaps down to Car­boniferous. So, the question as to which of them is more primitive has been often argued. Araucariaceae no doubt resembles the Cordaitales in its secondaiy wood and foliage. Its male gametophyte and embryo are also more primitive than that of Pinaceae.

A direct origin has been suggested on these grounds. But Jeffrey (1917) considers the wood of the Pinaceae more directly related to the Cordaitales and suggests that the similarity with Araucariaceae is a later development. He supposes (Fig. 737A) that the Araucariaceae evolved out of the Pinaceae at a very early stage.

The fusion of the ovuliferous and bract scales evolved at that stage led to the Araucariaceae. The other families arose later as shown in the diagram. Takhtajan (1953) more or less adheres to a similar view incorporating the findings of Florin (Fig. 737B).

The views of Seward (1905) about the Lycopodean origin of Araucariaceae and of Pennhallow (1904, 1907) that the Pinanceae originates from the Taxaccae which came from the Cordaitales no longer attract much attention. Lam (1955) suggests the origin of the Coniferopsida from a separate Lycopsida stock. This view also does not seem to be very much supported.

The position of the Taxaceae (Taxales) has been even more problematic. For­merly included within the Coniferales, it was recognised that the family shows no sign of a strobiloid ancestor although it may be clearly traced to the Upper Triassic and probably even earlier. It has been attempted to derive the family out of the Podo­carpaceae stock by Jeffrey as well as Takhtajan.

This does not seem to be very satis­factory. So Sahni (1920) suggested the separation of the Taxaceae from the Coniferales.

This has also been the view of Pilger in Engler’s Syllabus (1916, 1954) and of Florin (1951). Another point of argument is whether Podocarpaceae and Cephalo- taxaceae also should come within the new order. It is safest, however, to consider Taxaceae alone. Pennh allow’s view of deriving the Taxaceae directly out of the Cor­daitales has been cited above.

At the present state, it is better to consider the Taxales as a separate but problematic order.

The affinities of the Coniferopsida with the Pteridospermopsida and the Cyca­dopsida seem to be very remote. Any connection does not seem probable in the Spermaphytic level though earlier relationship is possible.