Terracotta pots

Flower pots made of terracotta (baked clay) are more expensive than plastic pots, but they provide a superior aeration to plant roots. Tetraploid

A cell or plant with four sets of chromosomes. A tetraploid usually develops from a more normal diploid , by an accidental doubling of its two sets of chromosomes. See also: Doubled monoploid, Haploid, Auto-polyploid, Allo-polyploid. Thea assamensis

Often called Camellia assamensis, this is the tea of India and Sri Lanka (Ceylon). It contains considerable germplasm from Thea sinensis, and it represents a vast hybrid swarm between these two species . Most commercial tea plantations are grown from true seed and the tea bushes show enormous variation, often with some 60% of the yield coming from about 30% of the bushes. Furthermore, the fermentation time of each bush is also variable, and this causes difficulties in the tea factory. The use of high-yielding, high quality clones eliminates these difficulties, and produces greatly increased yield and quality.

However, a tea plantation is usually good for a hundred years, and replanting is an expensive business.

The selection of tea clones is an appropriate task for amateur breeders . The should select the best yielding bushes out of a large area of seedling tea (up to one million bushes), and these selections are gradually reduced in number with a series of increasingly complex tests. Thea sinensis

Often called Camellia sinensis, this is the tea of China. See also: Thea assamensis. Theobroma cacao

The tree is often called cacao, while the product is called cocoa, from which chocolate is manufactured. But it is entirely correct to call them both cocoa. Note that cocoa was originally spelled coco, as in coconut and coco-yam, and the 'a' was added as a printing error in Johnson's dictionary. The old-fashioned spelling 'cocoanut' is incorrect.

The centre of origin of cocoa is on the eastern equatorial slopes of the Andes, and cocoa occurs throughout the Amazon Valley where it provides an interesting example of a cline. All the wild trees in the centre of origin are self-incompatible. As one moves down the Amazon, self-compatible types become increasingly common and, at the river mouth, they are all self-compatible.

All the cocoa in West Africa is self-compatible and very uniform, with a very narrow genetic base. When a very destructive African virus, called 'swollen shoot', appeared, the Government introduced a very unpopular eradication program which was not effective. In those days, horizontal resistance was not recognised. There is now considerable scope for a university breeding club to test buds of carefully quarantined foreign material grafted on to virus-infected trees. It should not be difficult to accumulate adequate horizontal resistance once the genetic base has been widened.

In Latin America, most cocoa populations are heterogeneous and are suitable targets for negative screening, with a view to eliminating the parasite interference coming from a few susceptible trees, in order to establish population immunity against witch's broom disease. Theoretical science

The agricultural sciences have been excessively empiricist during the twentieth century, and the theoretical aspects of these agricultural studies have been neglected. Good science must have a nice balance of both facts and ideas. One of the many advantages of theoretical science is its ability to predict novelty. For example, when Dimitri Mendeleev developed the periodic table, he was able to predict the existence of chemical elements that had not yet been discovered. Similarly, the Person-Habgood differential interaction can predict new vertical resistances and vertical parasitic abilities that have not yet been discovered. The n/2 model is also a prediction of novelty based on theoretical science. This model has yet to be shown to occur in nature but, if this demonstration is made, it will provide an elegant example of the powers of prediction of theoretical science.

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