In this article we will discuss about Taxus. After reading this article you will learn about: 1. History of Taxus 2. Morphological Features of Taxus 3. Internal Structures 4. Reproduction 5. Gametophytes 6. Embryogeny 7. Seed 8. Economic Importance.
Contents:
- History of Taxus
- Morphological Features of Taxus
- Internal Structures of Taxus
- Reproduction of Taxus
- Gametophytes of Taxus
- Embryogeny of Taxus
- Seed of Taxus
- Economic Importance of Taxus
Contents
1. History of Taxus:
Engler and Prantl (1889) recognized only one species, Taxus baccata, under Taxus, while Dallimore and Jackson (1948) and Sporne (1965) included 9 species viz. Taxus baccata, T. brevifolia, T. canadensis, T. chinensis, T. cuspidata, T.JIoridana, T. globosa, hunnewelliana and T. media. Taxus is widely represented in North and South America, Europe and Philippines, Algeria, Morocco and India extending even up to Malaysia.
In India, Taxus occurs both in the eastern as well as western Himalayas in Khasi and Naga hills, Assam, Manipur, Simla and several other areas at an altitude of about 1800 metres or more above sea level. Raizada and Sahni (1960) recognized a tenth species of Taxus (T. wallichiana) growing in Himalayas.
2. Morphological Features of Taxus:
Taxus baccata (Fig. 12.1), commonly known as ‘Yew’, is an evergreen tree attaining a height of 9-20 metres with a massive trunk. The stem is profusely branched and remains covered with a thin brown-coloured bark.
It differs from Pinus in not possessing dimorphic branches. All the branches are of unlimited growth and form a very dense canopy, thus making Taxus a shade-providing tree. Only the green leaves are present on the vegetative branches.
The leaves are linear, small, only 2-3 cm. long and spirally arranged (Fig. 12.1). Each leaf possesses a single strong vein and recurved margins. The upper surface is dark green while the lower surface is pale or rusty red in colour.
The apex is sharply pointed mainly because of accumulation of silica. This sharply pointed apex may cause death of catties eating these leaves. Each leaf is shortly stalked. The stalk broadens into a flat persistent base which shows a slight twist. The scaly leaves present on the fertile shoot are opposite and decussate. Taxus possesses a long and well- developed tap-root. The roots are deep-feeders and highly branched.
3. Internal Structures of Taxus:
(i) Stem:
In transverse section the stem is irregular in outline (Fig. 12.2) and resembles Pinus in structure. It is surrounded by a thickly cuticularised single-layered epidermis. Inner to the epidermis is parenchymatous cortex having some tannin-filled cells. It is followed by endodermis and sclerenchymatous pericycle.
The young stem shows a ring of conjoint, collateral, open and endarch vascular bundles enclosing a distinct pith in the centre. The protoxylem consists of spiral tracheids, and the phloem contains sieve cells with sieve plates and phloem parenchyma. Companion cells are absent.
The cambium is persistent and develops a thick vascular cylinder due to secondary growth. The cambium cuts secondary phloem towards outer side and secondary xylem towards inner side (Fig. 12.3). The secondary wood is devoid of resin canals and wood parenchyma. Its tracheids show uniseriate bordered pits only on their radial walls.
The tracheids also show spiral thickenings (Fig. 12.4). The medullary rays are uniseriate and homogeneous but in Taxus baccata they are sometimes bi-senate (Fig. 12.4). The wood is strong and dense. Due to the presence of tertiary spirals the wood is elastic in nature. Phellogen may develop in the older stems showing extrastelar secondary’ growth.
(ii) Root:
Except that of the absence of resin canals, the root anatomy of Taxus resembles very much with that of Pinus. The root is diarch.
(iii) Leaf:
The leaf (Fig. 12.5) is dorsiventral. It shows xerophytic characters. Upper and lower epidermal cells are rectangular in shape and thickly circularized. The cuticle is comparatively thin on the lower surface. The stomata are of sunken type and restricted only to the lower epidermis. They are haplocheilic in development.
The mesophyll is differentiated into palisade and spongy-parenchyma. The palisade is generally two-layered. Only one vascular bundle is present in the mid-rib region. Enclosed by a distinct endodermal layer or bundle sheath the collateral vascular bundle contains phloem towards the lower side and xylem towards the upper side.
Transfusion tissue is present on both the sides of the vascular bundle. Resin canals are generally absent. The xerophytic characters of the leaf include the presence of thick cuticle, sunken stomata, transfusion tissue and differentiation of mesophyll into palisade and spongy parenchyma.
4. Reproduction of Taxus:
Taxus is usually dioecious, but occasionally monoecious trees are also reported. The reproductive structures become prominent on the plant in February-March. The male and female plants do not show any distinction in their vegetative organisation, and the differentiation between them can be made only when the plants are in the flowering or fruiting stage. Vegetative reproduction in Taxus is not known.
(i) Male Strobilus or Male Flower:
The ‘male flowers’ or ‘male strobili’ are usually yellowish in colour and develop in the axil of foliage leaves (Figs 12.6 and 12.7). Each strobilus contains a number of overlapping sterile bracts Some of the bracts towards the tip of the strobilus are replaced by stamens or micro-sporangiophores.
Each stamen is shortly-stalked any has a peltate disc bearing 4-8 pendant microsporangia. The microsporangia surround the stalk completely (Fig. 12.7 C). The axis of the male strobilus contains a broad apex which is consumed in the formation of a stamen.
The microsporangia in the young male strobilus are compactly arranged but at maturity they get loosened and undergo dehiscence. The presence of peltate micro-sporangiophores is one of the most remarkable features of Taxus.
A mature microsporangium remains surrounded by an epidermal layer followed by two wall layers and sporogenous tissue. The outermost sporogenous cells differentiate into a tapetum (Fig. 12.8). Pennell and Bell (1986) has also observed a peritapetal membrane with orbicules in Taxus baccata.
The sporogenous cells start to behave as microspore mother cells which undergo meiosis and form microspores or pollen grains. The microspores remain arranged isobilaterally or tetrahedrally for quite sometime.
The development of microsporangium is of eusporangiate type (Fig 12.8) and is identical with that of Pinus. Four to eight archesporial cells develop hypo-dermally. They divide and form wall layers and sporogenous tissue.
In Taxus canadensis the microsporangium develops from one or rarely two hypodermal archesporial cells. Each microspore remains surrounded by two layers, i.e. intine and exine. It is uniaperturate, spheroidal and contains a mass of cytoplasm and a centrally located nucleus.
(ii) Female Strobilus or Female Flower:
The female strobili in Taxus are so highly reduced that they hardly appear as cones or strobili. They arise in the axils of leaves (Fig. 12.9) early in the season and mature in the next season.
Each female reproductive organ consists of a short primary axis having scaly leaves or bracts arranged in opposite decussate manner. A short secondary axis develops from the axil of upper three scaly leaves.
This secondary axis bears a few pairs of scaly leaves and a terminal ovule (Figs. 12.10, 12.11). According to Dupler (1920) the primary axis functions only as a vegetative branch of limited growth bearing the reproductive secondary axis.
1. Ovule:
The ovule is somewhat rounded or oval in shape and orthotropous. A single thick integument is present. Integument is free from the nucellus right up to its base forming a long micropyle. The integument is differentiated into outer fleshy, middle stony and inner fleshy layers. Two vascular strands enter the integument from the base of the ovule and reach up to its top.
A ring-like outgrowth develops from the base of the integument. It surrounds the entire ovule. It is called ‘aril’ or ‘cupule’ (Fig. 12.12). Aril is green and saucer-shaped when young but at maturity it is red and cup-shaped.
The aril also receives two vascular bundles but they are very minute and rudimentary. Pollen chamber and nucellar beak are absent in Taxus. The apex of the female gametophyte changes into a flask-shaped structure called tent-pole. The tent-pole disappears in the later stages.
Generally, up to 10 archegonia develop in the female gametophyte but sometimes as many as 25 archegonia may appear. In the young ovule an archesporial initial develops hypo-dermaly in the nucellus (Fig 12.13 A).
It divides penclinally forming an outer parietal cell and an inner primary sporogenous cell (Fig. 12.13 B). The latter divides again to form many sporogenous cells, of which generally one or more start to behave as megaspore mother cells.
The megaspore mother cell divides meiotically to form four megaspores which remain arranged in a linear tetrad (Fig. 12.13C,D). Generally, the lowermost megaspore remains functional and the remaining three degenerate (Fig. 12.13D). The functional megaspore nucleus divides, re-divides and develops ultimately into a multicellular gametophyte (Fig. 12.13 E,F).
2. Morphological Nature of the Aril:
The morphological nature of the aril has been quite controversial. Strasburger (1872) considered aril as an expansion of the axis. Sinnott (1913) compared the aril of Taxus with that of the epimatium of Podocarpaceous ovules while Jager (1899) considered aril as the second integument. According to Dupler (1920) the aril is equivalent to an outer fleshy layer in Taxus.
5. Gametophytes in Taxus:
(i) Male Gametophyte:
A microspore develops into a male gametophyte. It is uninucleate at the time of shedding. Dispersal of the microspores takes place by wind. A few microspores are taken up to the micropyle by the wind, where they are caught into the pollination drop. Through this drop the microspores are taken up to nucellus where they germinate.
At the time of germination, the microspore nucleus (Fig. 12.14A) divides and forms a tube cell and a generative cell (Fig. 12.14B). The exine ruptures and the intine comes out to form a pollen tube. The tube nucleus moves towards the tip of the pollen tube. The generative cell divides soon into a stalk cell and a body cell (Figs. 12.14C,D).
In the later stages two unequal male gametes are formed by the division of the body cell (Fig. 12.14E). Rohr (1973) has studied the formation of male gametes of Taxus in vitro conditions. He noticed the gametes to be similar, unlike in nature where they are unequal. The pollen tube reaches up to the archegonial neck by penetrating the nucellus. The prothallial cells are absent in Taxus.
(ii) Female Gametophyte:
The functional megaspore develops into the female gametophyte. It enlarges in size and its nucleus divides by many free-nuclear divisions to form as many as 256 nuclei. In Taxus baccata only 128 free-nuclei are formed according to Favre-Duchartre (1958).
A central vacuole develops and thus the free nuclei become panetal in position. There is a centnpetal wall formation as in Pinaceae and ultimately the whole of the tissue becomes cellular.
Certain archegonial initials are differentiated towards the micropylar end of the cellular female gametophyte. As already mentioned, as many as 25 archegonia develop in some cases. The development of archegonium in Taxus is similar with that of Pinus.
Each archegonium contains 2 to 4 neck cells and a large venter containing an egg nucleus (Fig. 12.15). There is no venter canal cell. The cytoplasm of the egg cell contains both small and large cytoplasmic inclusions and a zone of mitochondria and lipid globules near the nucleus.
(iii) Fertilization:
The tip of the pollen tube (Fig. 12.16), present near the neck of the archegonium, bursts, and both the male gametes, along with stalk nucleus and tube nucleus, are liberated into the archegonial venter (Fig. 12.17). The functional male nucleus (larger one) and the egg nucleus (Fig. 12.18) fuse and result in the formation of a zygote.
The other three nuclei (i.e. stalk nucleus, tube nucleus and smaller male cell) degenerate. As there are several archegonia in the female gametophyte, several eggs may be fertilized, resulting into a simple polyembryony. But ultimately only a single embryo attains maturity. The process of fertilization in Taxus canadensis has been studied by Dupler (1917).
6. Embryogeny in Taxus:
The zygotic nucleus divides to form 16 to 32 nuclei. Only 16 free-nuclei are formed in Taxus baccata (Fig. 12.19) according to Sterling (1948). Cell formation starts and soon the complete structure becomes cellular. A few cells at the tip of this pro-embryo develop into the embryo while the cells above it elongate and form suspensor.
The embryo gradually absorbs all the endosperm on development, and the seed thus becomes non-endospermic. The fully mature embryo is orthotropous and dicotyledonous. Only one embryo matures in one ovule.
7. Seed of Taxus:
Taxus seeds (Figs. 12.20, 12.21) are covered by a three-layered seed coat. The outermost layer is thin, brown and detaches soon. The middle layer is hard and stony while the innermost layer is fleshy. The mature seeds are covered by a red coloured aril. The aril serves to attract birds and help in dissemination.
The germination of seed is hypogeal. The seed germinates into a seedling possessing two cotyledonary leaves. The foliage leaves arise in the coming year. The cotyledonary leaves are larger than the normal foliage leaves.
8. Economic Importance of Taxus:
Besides its cultivation as an ornamental plant, the timber of Taxus baccata and some other species is oily and heaviest of the soft woods. It is quite durable and used for making decorative veneers, turnery, flowers and posts. Several other types of decorative pieces are also made from its wood.
Due to the presence of a toxic alkaloid (taxine) in the leaves, shoots and seeds, the entire plant of Taxus baccata is considered poisonous and also used in some countries as a fish poison. Its leaves also contain traces of ephedrine, and, therefore, used in bronchitis, asthama and epilepsy. Seeds of T. baccata are used as sedative.