(a) Give an account of post-fertilization changes leading to formation of seeds in angiosperms. 10 (b) What are the various…

(a) Post-Fertilization Changes in Angiosperms

Post-fertilization events in angiosperms are a coordinated sequence leading to seed formation. Following double fertilization, the zygote develops into the embryo, while the secondary nucleus forms the endosperm. Endosperm development varies: in nuclear type (e.g., Zea), free nuclei divide without walls; in cellular type (e.g., Datura), walls form immediately; and in helobial type (e.g., Eremurus), the first division is followed by a transverse wall creating a large micropylar and a small chalazal chamber, in which free-nuclear divisions follow. The embryo develops from the proembryo stage through globular, heart, torpedo, and mature stages, establishing the apical meristem, cotyledons, and radicle. Simultaneously, the integuments differentiate into the seed coat (testa), providing protection. The ovary wall matures into the fruit pericarp. The nucellus is usually consumed by the growing embryo and endosperm, but persists as perisperm in black pepper and beet; the funicle scar becomes the hilum, the micropyle persists as a pore, and the seed accumulates reserves, loses water and enters dormancy. These changes transform the ovule into a dispersal unit, ensuring the survival and propagation of the next generation.

(b) Anomalous Secondary Growth in Dicots

Anomalous secondary growth occurs when vascular cambium activity deviates from the standard pattern of concentric rings. In included phloem, as seen in Salvadora and Leptadenia, the cambium produces phloem internally, trapping it within the xylem. In medullary vascular bundles, found in Piper and Amaranthus, vascular bundles originate in the pith, creating a ring of secondary xylem and phloem within the central pith. Successive cambia are characteristic of Chenopodium and Boerhaavia, where new cambial rings form outside the previous ring of phloem. Each new cambium produces centripetal xylem (inward) and centrifugal phloem (outward), resulting in multiple concentric rings of xylem and phloem. In accessory cambium, as in Bougainvillea, a supernumerary cambium arises outside the phloem of the normal ring and cuts off vascular bundles embedded in lignified conjunctive tissue. Finally, abnormal activity of normal cambium, seen in Bignonia, involves four cambial segments producing more phloem and less xylem, so phloem wedges furrow the xylem in a cross-like pattern. These anomalies complicate wood structure but are key diagnostic features for identification.

(c) Indian Botanical Gardens and Their Role

Three prominent Indian botanical gardens are the Acharya Jagadish Chandra Bose Indian Botanic Garden in Howrah (under the Botanical Survey of India, with the Central National Herbarium), the Lloyd Botanic Garden in Darjeeling, and the Botanic Garden of the CSIR-National Botanical Research Institute (NBRI) in Lucknow. Lalbagh in Bengaluru, the Government Botanical Garden in Ooty and the JNTBGRI garden at Palode, Thiruvananthapuram, are also acceptable. These institutions play a pivotal role in modern plant research. Firstly, they serve as critical ex-situ conservation hubs, maintaining living collections of endangered and threatened species. Secondly, they act as germplasm repositories, preserving genetic diversity for future breeding programs. Thirdly, they facilitate taxonomic research by housing herbaria with type specimens, aiding in the classification and nomenclature of Indian flora. Fourthly, they support bioprospecting for medicinal and industrial compounds, linking traditional knowledge with modern pharmacology. Lastly, they engage in public education, raising awareness about biodiversity and environmental sustainability. These gardens are thus indispensable for balancing conservation, research, and societal engagement in plant sciences.

(d) Botanical Names and Families of Medicinal Plants

• Aconite: Aconitum napellus (the classical European source; the Indian drug comes from A. ferox and A. heterophyllum), family Ranunculaceae. The tuberous roots contain the highly toxic alkaloid aconitine; it is used in liniments for neuralgia and, only after traditional purification, in Ayurvedic formulations.
• Isabgol: Plantago ovata, family Plantaginaceae. The seed husk (psyllium) is a mucilaginous bulk laxative; India is the leading producer, mainly from Gujarat and Rajasthan.
• Ashwagandha: Withania somnifera, family Solanaceae. The roots, rich in withanolides, are used as an adaptogen and general tonic.
• Vasaka: Justicia adhatoda (syn. Adhatoda vasica), family Acanthaceae. The leaves yield the alkaloid vasicine, the source of the expectorant bromhexine, used for cough and bronchitis.
• Ipecac: Carapichea ipecacuanha (syn. Cephaelis ipecacuanha), family Rubiaceae. The roots contain emetine and cephaeline, used as an emetic and an anti-amoebic.

(e) Embryo Rescue and Crop Improvement

Embryo rescue is the aseptic excision of an immature or weak embryo from a developing seed or fruit and its culture on a nutrient medium in vitro, so that an embryo which would otherwise abort grows into a plant. It is most often needed when the endosperm fails after fertilisation, as in wide crosses.

It helps crop improvement in several ways. It recovers interspecific and intergeneric hybrids that fail through post-zygotic barriers such as endosperm breakdown; wheat × rye (triticale), wild Oryza × cultivated rice crosses for disease resistance, and Brassica and cotton wide crosses are examples. It is used to raise haploids after chromosome elimination (for example barley × Hordeum bulbosum and wheat × maize crosses), which then give doubled haploids. It rescues embryos of early-ripening fruits such as peach, cherry and plum, and of seedless grapes, where the embryo aborts before the fruit ripens, and it helps obtain triploid hybrids. It also shortens the breeding cycle by bypassing seed dormancy and slow seed maturation.

Its two main disadvantages are these. First, success is genotype-dependent and often low: very young, heart-stage or earlier embryos need complex, species-specific media, and many fail or germinate precociously into weak seedlings. Second, it is labour-intensive and costly, needing aseptic laboratory facilities and skilled staff, so only a few hybrid plants are recovered per cross. Where a callus phase intervenes, somaclonal variation can add unwanted changes.