Botany and Plant Science

Cytokinin is a plant hormone that is synthesized in the root. It shows acropetal transport. It is exactly opposite to the basipetal transport of auxin hormone. Acropetal transport is the translocation of hormone towards the shoot apex.

Gossypium hirsutum is the most widely cultivated cotton species and is known as a New World tetraploid cotton.

Chlorellin, contained in chlorella, acts like a natural antibiotic. In contrast to chemical antibiotics, it combats pathogenic bacteria without altering the intestinal flora.

Genera like Selaginella and Salvinia which produce two kinds of spores, macro (large) and small (micro) spores are known as heterosporous. Microspore and macrospore germinate and gives rise to male and female gametophyte respectively. The female gametophytes in these plants are retained on the parent sporophyte for a variable period. The development of the zygote into young embryo takes place within the female gametophytes. This event is a precursor to the seed habits considered an important step in evolution. So, the correct answer is 'Embryo develops in female gametophyte which is retained on parent sporophyte'.

Selaginella is a pteridophyte also known as spike moss or little club moss. It is a tropical plant. It has world wide distribution. It grows in damp forests. Some species occur in temperate regions. They grow in moist shady places. In Selaginella male gametes are spindle shaped and biflagellate. Hence option D is correct.

Selaginella exhibits heterospory, endosporic development, reduced gametophytes, and strobilus formation, which are considered advanced evolutionary traits in pteridophytes.

Heterosporous pteridophytes (e.g., Selaginella) always produce dioecious gametophytes. A heterosporous life history occurs in some pteridophytes and in all seed plants. It is characterized by morphologically dissimilar spores produced from two types of sporangia: microspores, or male spores, and megaspores (macrospores), or female spores. In pteridophytes, megaspores are typically larger than microspores, but the opposite is true in most seed plants.

In selaginella, the sporangia near the apex of a vertically-held strobilus or along the upper side of a horizontally-held strobilus are darker in color and are oval. These contain many very tiny orange spores, the products of meiosis of several sporocytes. These nearly-microscopic spores are called microspores. This makes their sporangium a microsporangium and the sporophyll a microsporophyll. The microsporangia is yellowish brown in color.

Selaginella rupestris or rock spike moss belongs to the family Selaginellaceae. The plant is heterosporous with the presence of both megaspores and microspores. However, the variation in sporangial distribution pattern in the strobili is related to the number of megaspores per megasporangium. More than often, the megasporangiate has only 1  megaspore per megasporangium.

• The sporangia produce spores by meiosis in spore mother cells. The spores germinate to give rise to inconspicuous, small but multicellular. free-living, mostly photosynthetic thalloid gametophytes called prothallus.
•  These gametophytes bear male and female sex organs called antheridia and archegonia, respectively. 
• The microspore germinates to form the male gametophyte. 
• The first unequal division (1-1) in a microspore results in the formation of a small lenticular prothallial cell (vegetative cell) and a large antheridial initial. The prothallial cell does not divide further. 
• Hence, prothallial cells in the male gametophyte of Selaginella is 1.
• So, the correct answer is '1'.
  

Pteridophytes are vascular plants. Leaves called sporophylls produce clusters of capsular structures called sporangia that bears spores which germinate and yield gametophytes. Hence they show alternation of generations, in which a sporophyte produces spores through meiosis and a gametophyte forms gametes by mitosis. 

Option D is mismatched as the sporangia in selaginella contain four very large yellow spores. These four are the result of meiosis from a single sporocyte. The spores are called megaspores because of their large size. This makes the sporangium a megasporangium and the sporophyll a megasporophyll.

Rhizophores is a  structure of controversial morphological nature.They are leafless and  positively geotropic organs that have localised origin and develop from angle meristems. Goebel and Bower considered them as Organs sui generis (i.e.,neither roots or shoots). According to J.C.Shoute,Organ sui generis is one that is a novelty and has not arisen as result of  metamorphosis of any organ.

In Dryopteris sori, a group of sporangia covered with indusium are present along the margin of pinnae or leaflets. In it 2-8 sori are borne on the abaxial surface of the mature fronds (sporophylls). They are situated in two rows at the tips of the veinlets in between the midrib and the margins. The sori appear reniform due to the kidney shaped protective thin and membranous covering, called as indusium.