Taxus Systematic Position and Structure

Taxus:-

Systematic Position:-

Division:- Coniferophyta

Class:- Coniferopsida

Order:- Taxales

Family:- Taxaceae

Genus:- Taxus

External Morphology:-

> Taxus baccata, 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. 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.

Internal Structure:-

a. Stem:-

> In transverse section the stem is irregular in outline 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. 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. The medullary rays are uniseriate and homogeneous but in Taxus baccata they are sometimes bi-senate. 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.

b. 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.

c. Leaf:-

> The leaf 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.