Botany and Plant Science

• Casuarina equisetifolia is a monoecious, wind-pollinated species that undergoes chalazogamy fertilization.
  
• In this type of fertilization, the pollen tube penetrates the ovule through the tissue of the chalaza.
  
• It was first observed by Treub in 1981 in Casuarina species.
  

The pollen tube consists of two male gametes. One male gamete fertilizes the egg cell placed in between the egg apparatus and forms the diploid oospore/zygote which forms embryo after maturation. While the other male gamete fuses with the two polar nuclei of the central cell and forms the triploid PEN (Primary Endosperm Nucleus) which results in the formation of endosperm after maturation.

Chemotaxis is the movement of an organism or plant in response to a chemical stimulus. Somatic cells, bacteria and other single cell or multicellular organisms direct their movements according to certain chemicals in their environment. It is clearly seen in the case of growth of pollen tube towards the embryo sac. The pollen tube does not directly reach the ovary in a straight line. It travels near the skin of the style and curls to the bottom of the ovary, then near the receptacle, it breaks through the ovule through the micropyle and the pollen tube bursts into the embryo sac.

In mesogamy, pollen tube enters the ovule through the integuments or funiculus. After the pollen tube enters the embryo sac, its apex dissolves and the male gametes are liberated. One of the male gametes are liberated. One of the male gametes fuses with the egg and produces a diploid zygote which by further division develops into an embryo. The other male gamete fuses with the diploid secondary nucleus and produces a triploid primary endosperm nucleus. This is called double fertilization.

Synergid cells are two specialized cells that lie adjacent to the egg cell in the female gametophyte of angiosperms and play an essential role in  pollen tube guidance and function. The term synergid comes from the Greek word ' Synergos' which means 'working together' and was reportedly coined by Eduard Strasburger. The pollen tube grows towards and into one of the synergid cells, which begins to degenerate as the pollen tube enters it. The pollen tube then ceases growth, ruptures and releases its two sperm cells which migrate to the egg and central cell, fertilization of which gives rise to the embryo and endosperm, respectively. The synergid cells are necessary for pollen tube guidance in the ovule.

Paratonic growth movements are the induced movements of growth, caused by external stimuli, such as gravity, light, water or contact. The movements show a directional relation to the stimulus. The stimuli are effective in causing growth movements only when they are unidirectional. Such movements are generally known as tropic movements and the phenomenon is referred to as tropism. 

Movements of locomotion are generally very fast movements, which may be
(A) Autonomic or spontaneous and
(B) induced or paratonic.
The induced locomotory movements are also called as tactic movements induced by external stimuli. Based on the nature of stimuli, these may be of three types:
(i) Phototactic movement: Seen in zoospores and gametes, which are provided with a light sensitive eye spot, which is attracted by low light intensities.
(ii) Thermotactic movement: Movement in response to heat stimulus is seen in certain motile algae, like Chlamydomonas, which moves from a colder to a warmer place.
(iii) Chemotactic movement: In bryophytes and pteridophytes, the swimming antherozoids are attracted towards the archegonium by chemical stimuli, such as organic substances, like sugar, malic acid and so on.

The pollen tube grows towards embryo sac because of influence of certain chemicals. Pollen tube most commonly enters embryo sac through micropyle and bursts in one of the synergid. It is believed that filiform apparatus of synergids secrete pollen tube attracting chemicals. Mineral; boron is required for germination of pollen grains and elongation of pollen tube. The growth of pollen tube is an example of chemotropism.

Geotropism is movement or growth with respect to gravitation force; it may be positive geotropism (in direction of gravity) or negative geotropism (away from gravity). This makes option A incorrect as growth of pollen tube does not occur in response to gravity. Phototaxis refers to movement/growth of plant or plant part in response to sunlight. This makes option C incorrect as growth of pollen tube occurs towards the embryo sac, not towards or away the sunlight. Thigmotaxis refers to unequal growth caused by contact of plant part with a solid surface which makes option D incorrect. A tropic response of plant or plant part towards a chemical stimulus is referred to as chemotaxis. Pollen tube growth towards the embryo sac is driven by ion flux (reactive oxygen species) and calcium gradient which makes option B correct. 

The eight nucleate female gametophytes of flowering plants have an egg apparatus at micropylar end which in turn has one egg cell and two flanking cells. The flanking cells are known as synergids and have finger like projections of wall in their cytoplasm, the filiform apparatus. They serve as site for entry of pollen tube into one of the synergid followed by disintegration of later. The pollen tube pierces through nucellus and traverses the wall of embryo sac between egg and synergid. Filiform apparatus guide the pollen tube to egg nucleus and thereby facilitate syngamy. This makes option A correct. Option B is incorrect as egg nucleus does not have filiform apparatus to guide the direct entry of pollen tube into it. Three cells present at chalazal end of embryo sac are called as antipodals which makes options C and D incorrect as antipodals and synergids are present at opposite ends of embryo sac. 

Siphonogamy is the condition in plants in which pollen tubes are developed for the transfer of the male cells to the eggs. The seed plants are termed as siphonogamous. As a consequence, the spermatophytes are called as siphonogamous by some botanists. Syngamy is the fusion of gametes producing in the formation of a zygote, which develops into a new organism. 

Porogamy is the entrance of the pollen tube in the seed plants through the micropyle. Whereas Chalazogamy is the process of fertilization in which the pollen tube penetrates to the embryo sac.

Therefore, the correct answer is option C.