Allopatric
(Describing the geographical relationship of different species or sub-species), not occurring together, i.e. having different areas of distribution.

Allopolyploid
A polyploid organism in which two different species have each contributed one or more sets of chromosomes. Cultivated wheats are probably allopolyploids.

All-or-none law
A type of response about certain irritable tissues, that they have in standardized conditions only two possible reactions to stimuli of whatever intensity; either no response, or response of a strength that cannot be varied by varying the strength of the stimulus. Applies to nerve cell in the generation of an impulse and to many muscle fibre in the generation of contraction.

In plants seen most clearly in, e.g., ferns where the two generation are independent. The fern plant is a diploid sporophyte and reproduces asexually by formation of haploid spores following meiosis. Germination, of the spores initiates the gametophyte generation, a small prothallus, which reproduces sexually. Male and female gametes fuse together to form a zygote which develops into a new fern plant.

Alluvial Soil
A young soil derived from river, estuarine or marine deposits, and having a high fertility.