Botany and Plant Science

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.

The entry of the pollen tube into the ovule through the chalaza is known as the chalazogamy. This was discovered by Treub. One such example of xerophyte is Casuarina, which is a plant which can withstand prolonged periods of drought. A flower is a bracteate, bracteolate, sessile, incomplete and unisexual.

Thus, the correct option is D.

Chemotropism is the directional growth movement in response to external chemical stimuli. Movement of pollen tube towards ovule is chemotropic movement. Haptotropism is the growth movement of plant part in response to the touch stimuli. Thigmotropism is the growth movement in response to the touch. Phototropism is the growth movement of plant part in response to the light.

After the germination of pollen grains on stigma pollen tube is produced through a germ pore . Pollen tube grows chemotropically through the tissue of stigma and style in response to sugary fluid.

The pollen tube wall is primarily composed of pectocellulose, which provides the necessary flexibility for growth.

The growth of the pollen tube towards the embryo sac is directed by chemical signals released by the synergids, a process known as chemotaxis.

A mature male gametophyte in angiosperms typically consists of three cells: one tube cell and two male gametes.

The pollen tube releases its male gametes into one of the two synergids, specifically the one that is degenerating.