Life Cycle of Taxus | Coniferopsida | Gymnosperms | Botany

In this article we will discuss about the life cycle of taxus.

The Sporophyte of Taxus:

The plant is a slow-growing tree with an usual height of 30- 40 feet, and bearing horizontally growing branches of unlimited growth forming a thick canopy. Dwarf shoots are absent. The leaves are simple, dorsiventral, slightly stalked, and are twisted at their bases, giving rise to a two-ranked arrangement. They contain silica particles, particularly so at the sharp-pointed tips. Scale leaves are absent.

Internally, the stem shows a general arrangement of the Pinus type, with the exception that the resin canals are entirely absent, and the tracheids of the secondary wood possess spiral thickenings in addition to the usual uniseriate bordered pits. The medullary rays are also uniseriate, and wood parenchymas are absent.

The root is diarch, and is also devoid of resin canals, and possesses spiral thickenings in the tracheids.

The internal structure of leaf resembles that of a leaflet of Cycas in some respects. There are two cuticularized epidermal layers, the upper one being continuous, while the lower one is interrupted at intervals by sunken stomata.

The mesophyll is differentiated into upper palisade and lower spongy parenchymas; sometimes the latter may contain some resinaceous, yellow-coloured matter within some cells. There is a solitary vascular bundle in the centre surrounded by an endodermal layer, with xylem towards the upper surface and phloem towards the lower one. The pericy­cle is modified into transfu­sion tissue.

Taxus is dioecious, and the male flowers are borne in strobili, while the female ones do not form any cone.

Staminate (or Male) Strobilus:

The staminate strobili are borne in the axils of leaves, produced on branches of the previous year. Each strobilus consists of a short central axis, bear­ing about a dozen of spirally arranged scales, in basipetal order of development, at their bases and a cluster of closely inserted, umbellate microsporophylls at the top. It is to be noted that the vegetative apex is also utilized in the production of sporophyll.

Each microsporophyll is a peltate, shield-like body with 6-8 pendant microsporangia, hanging from the under-surface of the shield. These sporangia are united with one another, as well as with the sporophyll-stalk. The microsporangia contain numerous microspore mother cells, each of which on undergoing reduction division, gives rise to a tetrad of microspores or pollen grain.

The pollen grains are liberated by the dehiscence of the sporangium, and are dispersed by the wind. They come in the neighbourhood of the ovule, and are caught by the pollination drop, exuded from the micropylar end of the ovule.

Megasporangium (or Ovule):

The ovules occupy the same axillary position on the female plants, just as the male cones do in the male ones. There is also a short central axis, which is known as the primary axis, bearing a number of closely imbricated, over­lapping, sterile scale leaves on its basal region.

In the axil of the uppermost scale there arises the fertile shoot, the secondary axis, which bears three pairs of decussately arranged minute scale leaves, the lowermost pair of which stands at right angles to the subtending bracts. This secondary axis apparently terminates in a solitary ovule. The vegetative apex of the primary shoot may occasionally proliferate, producing another secondary axis, bearing an ovule in the next year.

The ovule is orthotropous. The nucellus is free from the integument. The latter is three-layered as usual and forms a prominent micropylar canal above the nucellus. In some cases, in the adult stage the nucellus and the integument may be fused. Just below the integument, there appears a slowly growing ring­-like structure, which later on surrounds the entire ovule.

This is usually known as the aril, but it cannot be compared to the structure bearing the same designation as is found in the angiosperms. On the contrary, it can be compared with the epimatium of the Podocarpus, while others consider it to be an outer integument. This structure is also referred to as the cupule by some author. With the maturity of the seed it becomes red, and gives the seed a characteristic berry-like appearance.

The Gametophytes of Taxus:

Male Gametophyte:

The microspore is the first cell of the male gametophyte. The deve­lopment of the male gametophyte begins at the pollination stage in the middle of March. The pollen grain is shed in a uninucleate stage, and the formation of any prothallial cell is completely eliminated.

Pollination takes place by the agency of wind. The division of the pollen-nucleus results into an antheridial cell, and a tube cell. The former divides later on into a stalk cell and a body cell. The body cell, by undergoing another division, gives rise to two un­equal cells, which finally develop into two dissimilar non-flagellate male gametes.

Female Gametophyte:

The megaspore is the first cell of the female gametophyte. The megaspore nucleus divides freely, and gives rise to 256 nuclei. The cell, in the meanwhile, enlarges rapidly, and these nuclei are placed peripherally around a central vacuole. Subsequent to this, wall-formation begins.

The walls are laid down at right angles to the megaspore membrane, and they extend to the middle of the megaspore cavity, giving rise to long tube-like cells known as the alveoli. In almost all cases, the centri­petal ends of the alveoli are provided with walls only at a later stage.  

Subsequently, further divisions of the alveoli take place, resulting in a large number of cells forming the en­dosperm (the female gametophyte). The cells of the endosperm are uninucleate at first, but later on the number of nuclei in each may multiply during the month of July.

Ultimately, the nuclei in each cell degenerate, and undergo fusion forming a single, large, irregular body. The archegonia, 5-8 in number, develop from the superficial cells at the top of the endosperm. The structure of the archegonium is rather simple. There are four neck cells, and only one egg cell in the venter; no ventral canal cell has yet been recorded.  

The interval between pollination and fertilization is a short period of one month only. The pollen tube, developed from the tube cell, reaches the female gametophyte at a period, when the latter may be in a comparatively early stage of development, or in other words, the archegonium-initial may not have yet been recognized.

The tip of the pollen tube finally ruptures, and all the four nuclei enter into the archegonium, but only the larger of the two male nuclei unites with the egg forming oospore, while the remaining three dege­nerate. Several arche­gonia may be thus fer­tilized, giving rise to a case of simple polyembryony, but normally only one em­bryo reaches maturity.

The New Sporophyte of Taxus:

The oospore undergoes a number of free nuclear divisions, resulting in 32 nuclei, which migrate to the base of the developing sporophyte. At this stage, walls begin to be formed separating the nuclei into cells, constituting the pro-embryo. The cells of the pro-embryo are arranged in three tiers. From the lowermost tier (which as a rule is composed of one cell only) the embryo is developed.

The other tiers, of cells give rise to the suspensor. The mature embryo is dicotyledonous, and in course of its attaining maturity, consumes the endosperm wholly. The aril of the mature seed, on account of its red colour, attracts the birds, and thus the seed is dispersed through their agency. The germination of the seed is hypogeal, and on germination, it gives rise to a seedling sporophyte.