See Fertilisation Sexual recombination

The recombining of genetic traits that occurs when a male gamete fuses with a female gamete. Shallot

See: Allium cepa. Shareware

Computer software that is copyrighted but that can be distributed free of charge to anyone. The author of shareware requests a voluntary donation from regular users who find the software useful. Shifting cultivation

A system of agriculture in areas of low population density. New land is cleared each season, and this provides better plant nutrition, and an escape from parasites. This system is also known as 'slash and burn'. Short-day

Many tropical plants are photoperiod-sensitive, and depend on a short, twelve-hour day to initiate flower production and, possibly, other processes, such as tuber formation. Equally many temperate plants depend on a long day to initiate these processes. Some plants are photoperiod-insensitive, or day-neutral. See also: potatoes. Shot-hole

A leaf disease in which the dead central portion of a roughly circular lesion falls out, leaving a hole. When a plant has many of these lesions, it looks as if it had been hit with a charge from a shotgun. Shrub

A woody, perennial plant that is too small to be called a tree. Sibling

In common usage, this term means brother or sister, without the gender being specified. In plant breeding, siblings are all the plants that come from one parent, and they are often referred to as 'sibs'. Full-sibs have the same male and female parents.

Half-sibs have the same female parent that was randomly cross-pollinated, and the male parents are thus unknown. Sigmoid growth curve

The seasonal, S-shaped, population growth curve of an r-strategists organism such as a crop parasite. Typically, the curve shows an initial slow growth (the lag phase), followed by a population explosion with logarithmic growth (the log phase), followed by a rapid slowdown (the leg phase) as environmental factors become unfavourable. This kind of population growth is usually followed by a population extinction. Silage

Green fodder crop material, such as grass or clover, that has been stored in a silo, or in a large plastic film tube, and allowed to ferment to become food for cattle. This is a chemical fermentation and it produces heat that both stops the fermentation and sterilises the silage.

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