Asexual reproduction perpetuates a species without any gametes ever being formed or fused, and it is the general mode used by lower plants, fungi, bacteria and many animals: conidia formation (Aspergillus, Penicillium), budding (yeast, Hydra), fragmentation (Spirogyra), gemma production (Marchantia), regeneration (Planaria) and binary fission (bacteria) are all textbook examples, and every individual produced this way is morphologically and genetically identical to its parent - collectively, such identical individuals are called clones. Higher (flowering) plants reproduce asexually mainly through vegetative reproduction, in which a bud on a root, stem or leaf grows and detaches to form a complete new plant, using organs that often double as storage/perennation structures; the basic unit of such propagation is called a reproductive propagule or diaspore. Natural vegetative reproduction needs no human intervention: roots (Murraya, Dalbergia, Millingtonia; tuberous roots in Ipomoea batatas and Dahlia), stems (rhizome in Zingiber/Musa/Curcuma, corm in Colocasia, tuber in Solanum tuberosum with axillary-bud 'eyes', tunicated bulb in Allium cepa, naked bulb in Lilium, runner in Centella asiatica, stolon in Fragaria, offset in Pistia/Eichhornia, sucker in Chrysanthemum, bulbil in Dioscorea/Agave) and leaves (epiphyllous buds in Bryophyllum and Scilla) all produce adventitious buds that detach and grow independently; this needs only one parent, gives genetically identical offspring, and can spread very fast (e.g. the invasive weed Eichhornia crassipes, the 'Terror of Bengal'), but introduces no genetic variation at all. Artificial (human-assisted) methods split into conventional techniques - cutting (root/stem/leaf pieces rooted in a medium), grafting (joining a rooted stock to a scion by bud, approach, tongue, crown or wedge grafting) and layering (rooting an attached stem, either by mound layering, bending a branch into buried soil, or air layering, girdling and wrapping a node in moist soil) - which give fast, uniform, disease-free-selectable propagation but risk transmitting infection and are bulky to handle; and modern methods, resting on F.C. Steward's 1932 discovery that a totipotent carrot phloem cell can regenerate a whole plant, which underlie tissue culture and micropropagation (regenerating a plant from a single cell/tissue/small vegetative piece), rapidly multiplying desired, disease-free, even rare or endangered genotypes in any season, though the process is labour-intensive, needs sterile conditions, and produces a genetically uniform crop equally vulnerable to any new disease.