Parenchyma

Plant tissue consisting of unspecialised cells, usually with air spaces between them. Many plant organs, such as the inside of many stems (e.g., pith), are made up mainly of parenchyma. Paris Green

A singularly nasty pesticide containing copper and arsenic, which was widely used until replaced with DDT. Parsley

See: Petroselinum crispum. Parsnips

See: Pastinaca sativa. Parthenocarpic

The production of fruit without pollination, as with bananas. Parthenogenetic

The development of an individual from a gamete without fertilisation. Partial resistance

This term, meaning 'incomplete', was originally used to describe horizontal resistance. Unfortunately, 'partial' also means biased, and the term would better describe vertical resistance. Horizontal resistance would then be impartial resistance. These terms are best avoided. Pascal's triangle

A mathematical device for calculating the possibilities of 'either-or' events, such as 'boy or girl' in single-child births, or the presence or absence of vertical genes. For example, with three births, there are one possibility of three boys, three possibilities of two boys and a girl (i.e., 'boy-boy-girl', 'boygirl-boy', and 'girl-boy-boy'), three possibilities of two girls and a boy, and one possibility of three girls. These possibilities are called the binomial coefficients. They are important for calculating the numbers of biochemical locks and keys that there will be in the n/2 model of the gene-for-gene relationship and the vertical subsystem. Paspalum spp.

Tropical fodder grasses from South America. Passiflora edulis

The passion flower, which is cultivated for its fruit that are used to add flavour to fruit salads and drinks. This crop is open-pollinated and offers scope for amateur breeders who should aim at horizontal resistance to locally important parasites, increased fruit size and juice content, and yield. Passion fruit

See: Passiflora edulis. Pasta wheat

See: Triticum durum. Pasteurisation

Named after Louis Pasteur, this is a technique of heating wine, milk, food, or soil to about 80°C in order to destroy harmful micro-organisms. This level of heating does not lead to a complete sterilisation, for which a temperature of about 120°C is required. Pasteurised soil can be used as soon as it is cool, whereas sterilised soil usually needs about three weeks to recover its beneficial micro-biological activity. Pastinaca sativa

The parsnip, which is an open-pollinated, biennial member of the Umbelliferae, and is cultivated for it large, yellow, tapering root, which is eaten as a vegetable. Amenable to breeding by amateurs.

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