Taxus baccata commonly known as ‘Yew’, is an evergreen tree attaining a height of 9-20 metres with a massive trunk. 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 organization. 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.
Diuretic, Hypoglycemic and Limit test of Heavy metals and Arsenic.-1.pdf
Taxus
1. St. Xavier’s College, Mahuadanr
Department of Botany Asst. Prof. Sr. Kaslin Juliat
Taxus
Morphological Features
1. Taxus baccata (Fig. 12.1), commonly
known as ‘Yew’, is an evergreen tree
attaining a height of 9-20 metres with a
massive trunk.
2. The stem is profusely branched and
remains covered with a thin brown-
coloured bark.
3. It differs from Pinus in not possessing
dimorphic branches.
4. All the branches are of unlimited growth
and form a very dense canopy, thus
making Taxus a shade-providing tree.
5. Only the green leaves are present on the
vegetative branches.
6. The leaves are linear, small, only 2-3 cm.
long and spirally arranged
7. Each leaf possesses a single strong vein
and recurved margins.
8. The upper surface is dark green while the
lower surface is pale or rusty red in
colour.
9. The apex is sharply pointed mainly because of accumulation of silica.
10.This sharply pointed apex may cause death of catties eating these leaves.
11.Each leaf is shortly stalked. The stalk broadens into a flat persistent base which
shows a slight twist.
12.The scaly leaves present on the fertile shoot are opposite and decussate.
13.Taxus possesses a long and well- developed tap-root.
14.The roots are deep-feeders and highly branched.
Internal Structures of Taxus
(i) Stem
1. In transverse section the stem is
irregular in outline and resembles
Pinus in structure.
2. It is surrounded by a thickly
cuticularised single-layered epidermis.
3. Inner to the epidermis is
parenchymatous cortex having some
tannin-filled cells.
2. St. Xavier’s College, Mahuadanr
Department of Botany Asst. Prof. Sr. Kaslin Juliat
4. It is followed by endodermis and
sclerenchymatous pericycle.
5. The young stem shows a ring of
conjoint, collateral, open and
endarch vascular bundles
enclosing a distinct pith in the
centre.
6. The protoxylem consists of spiral
tracheids, and the phloem
contains sieve cells with sieve
plates and phloem parenchyma.
7. Companion cells are absent.
8. The cambium is persistent and
develops a thick vascular
cylinder due to secondary
growth.
9. The cambium cuts secondary
phloem towards outer side and
secondary xylem towards inner side .
10.The secondary wood is devoid of resin
canals and wood parenchyma.
11.Its tracheids show uniseriate bordered
pits only on their radial walls.
12.The tracheids also show spiral
thickenings.
13.The medullary rays are uniseriate and
homogeneous but in Taxus baccata
they are sometimes bi-senate
14.The wood is strong and dense.
15.Due to the presence of tertiary spirals the wood is elastic in nature.
16.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
1. The leaf is dorsiventral.
2. It shows xerophytic characters.
3. Upper and lower epidermal cells are rectangular in shape and thickly
circularized.
4. The cuticle is comparatively thin on the lower surface.
5. The stomata are of sunken type and restricted only to the lower epidermis.
3. St. Xavier’s College, Mahuadanr
Department of Botany Asst. Prof. Sr. Kaslin Juliat
6. They are haplocheilic in development.
7. The mesophyll is differentiated into palisade and spongy-parenchyma.
8. The palisade is generally two-layered.
9. Only one vascular bundle is present in the mid-rib region.
10.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.
11.Transfusion tissue is present on both the sides of the vascular bundle.
12.Resin canals are generally absent.
13.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.
Reproduction
1. Taxus is usually dioecious, but occasionally monoecious trees are also
reported.
2. The reproductive structures become prominent on the plant in February-March.
3. The male and female plants do not show any distinction in their vegetative
organization.
4. 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
Each strobilus contains a number of overlapping
sterile bracts Some of the bracts towards the tip of the
4. St. Xavier’s College, Mahuadanr
Department of Botany Asst. Prof. Sr. Kaslin Juliat
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.
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.
10 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 and is identical
with that of Pinus.
1. Four to eight archesporial cells develop hypo-dermally.
2. They divide and form wall layers and sporogenous tissue.
5. St. Xavier’s College, Mahuadanr
Department of Botany Asst. Prof. Sr. Kaslin Juliat
3. In Taxus canadensis the microsporangium develops from one or rarely two
hypodermal archesporial cells.
4. Each microspore remains surrounded by two layers, i.e. intine and exine.
5. It is uniaperturate, spheroidal and contains a mass of cytoplasm and a centrally
located nucleus.
(ii) Female Strobilus or Female Flower
1. The female strobili in Taxus are so highly reduced that
they hardly appear as cones or strobili.
2. They arise in the axils of leaves early in the season and
mature in the next season.
3. Each female reproductive organ consists of a short primary
axis having scaly leaves or bracts arranged in opposite
decussate manner.
4. A short secondary axis develops from the axil of upper
three scaly leaves.
5. This secondary axis bears a few pairs of scaly leaves and a
terminal ovule .
6. According to Dupler (1920) the primary axis functions
only as a vegetative branch of limited growth bearing the reproductive
secondary axis.
Ovule
1. The ovule is somewhat rounded or oval in shape and orthotropous.
2. A single thick integument is present.
3. Integument is free from the nucellus right up to its base forming a long
micropyle.
4. The integument is differentiated into outer fleshy, middle stony and inner
fleshy layers.
5. Two vascular strands enter the integument from the base of the ovule and reach
up to its top.
6. St. Xavier’s College, Mahuadanr
Department of Botany Asst. Prof. Sr. Kaslin Juliat
6. A ring-like outgrowth develops from the base of the integument.
7. It surrounds the entire ovule.
8. It is called ‘aril’ or ‘cupule’
9. Aril is green and saucer-shaped when young but at maturity it is red and cup-
shaped.
10.The aril also receives two vascular bundles but they are very minute and
rudimentary.
11.Pollen chamber and nucellar beak are absent in Taxus.
12.The apex of the female gametophyte changes into a flask-shaped structure
called tent-pole.
13.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.
It divides penclinally forming an outer
parietal cell and an inner primary
sporogenous cell
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
7. St. Xavier’s College, Mahuadanr
Department of Botany Asst. Prof. Sr. Kaslin Juliat
which remain arranged in a linear tetrad.
Generally, the lowermost megaspore remains functional and the remaining
three degenerate
The functional megaspore nucleus divides, re-divides and develops ultimately
into a multicellular gametophyte.
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 divides and forms a
tube cell and a generative cell.
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 .
In the later stages two unequal male gametes are formed by the division of the
body cell
The prothallial cells are absent in Taxus.
(ii) Female Gametophyte
1. The functional megaspore develops into the
female gametophyte.
2. It enlarges in size and its nucleus divides by
many free-nuclear divisions to form as many as
256 nuclei.
3. In Taxus baccata only 128 free-nuclei are
formed according to Favre-Duchartre (1958).
4. A central vacuole develops and thus the free
nuclei become panetal in position.
8. St. Xavier’s College, Mahuadanr
Department of Botany Asst. Prof. Sr. Kaslin Juliat
5. There is a centnpetal wall formation as in Pinaceae and ultimately the whole of
the tissue becomes cellular.
6. Certain archegonial initials are differentiated towards the micropylar end of the
cellular female gametophyte.
7. As already mentioned, as many as 25 archegonia develop in some cases.
8. The development of archegonium in Taxus is similar with that of Pinus.
9. Each archegonium contains 2 to 4 neck cells and a large venter containing an
egg nucleus
10.There is no venter canal cell.
11.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:
1. The tip of the pollen tube 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
.
2. The functional male nucleus (larger one) and the
egg nucleus fuse and result in the formation of a
zygote.
3. The other three nuclei (i.e. stalk nucleus, tube nucleus and smaller male cell)
degenerate.
4. As there are several archegonia in the female gametophyte, several eggs may
be fertilized, resulting into a simple polyembryony.
5. But ultimately only a single embryo attains maturity.
Embryogeny in Taxus
1. The zygotic nucleus divides to form 16 to 32
nuclei.
2. Cell formation starts and soon the complete
structure becomes cellular.
3. A few cells at the tip of this pro-embryo
develop into the embryo while the cells
above it elongate and form suspensor.
4. The embryo gradually absorbs all the
endosperm on development, and the seed
thus becomes non-endospermic.
5. The fully mature embryo is orthotropous and
dicotyledonous.
6. Only one embryo matures in one ovule.
Seed of Taxus
1. Taxus seeds are covered by a three-layered seed
coat.
9. St. Xavier’s College, Mahuadanr
Department of Botany Asst. Prof. Sr. Kaslin Juliat
2. The outermost layer is thin, brown and detaches soon.
3. The middle layer is hard and stony while the innermost layer is fleshy.
4. The mature seeds are covered by a red coloured aril.
5. The aril serves to attract birds and help in dissemination.
Germination of Seed
1. The germination of seed is hypogeal.
2. The seed germinates into a seedling possessing two
cotyledonary leaves.
3. The foliage leaves arise in the coming year.
4. The cotyledonary leaves are larger than the normal
foliage leaves.
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.
