The identifying characters described and used in this publication are found only on the external surface of the seeds. Their usefulness for identification varies. Characters of major importance are colour, size and shape of the seed and nature of the seed coat. Other characters used in conjunction with these features have limited use. These include the hilum or seed scar and radicle.
Colour—Seeds may vary from pale to dark yellow in yellow domestic mustard seed; dark yellow to tan with infrequent brownish seeds in oriental mustard seed; varying shades of reddish-brown in brown mustard seed; varying shades of reddish-brown to grayish-black and black in common wild mustard seed and rapeseed; shades of yellow to reddish-brown to grey-black or nearly black in canola depending on the canola varieties. Lack of maturity may affect the true colour of the seeds. Colour is one of the most important characteristics used to identify species of Brassica and Sinapis. For example, yellow mustard seed, oriental mustard seed and rapeseed in bulk are distinguishable from each other by their colour.
Size and Shape—See Figure 1. The size of small oilseeds varies from the larger seed of domestic yellow mustard seed and some varieties of Brassica napus to the smaller seeds of brown mustard. There is considerable overlapping in this characteristic in the different classes but it is sufficiently consistent to be a very useful characteristic in identifying some species of Brassica and Sinapis. Weed seeds commonly found in these oilseeds also vary significantly in their size and shape.
The shape of the seeds of some oilseed classes or their varieties is consistently spherical; others are oblong or oval. Some have a longitudinal groove or depression with a ridge along the length of the centre of the groove associated with the position of the radicle; others may be flattened at one or both ends. The shape may vary in immature or poorly developed seed but is a reasonably useful characteristic in the identification of well-developed mature seed, particularly when applied to common wild mustard seed and canola/rapeseed.
In this manual drawings are enlarged and therefore the size depicted is not comparable from seed to seed.
The following pictograph provides approximate relational sizes of some seeds to help the user understand the differences in size.
Seed Coat—This is the outer protective covering of the seed. Useful identification features include the reticulations on the seed coat, the nature of the stippling and the relative size of the interspaces.
The reticulations are the network of ridges that appear on the seed coat. They may vary in prominence and arrangement. In some cases the pattern may resemble lace or netting; in others the lines may be more parallel and may radiate from the axis of the seed in a sunburst pattern and others may not appear to have any definite pattern. The prominence and texture of the ridges must be considered in conjunction with the pattern of the reticulations. They may be sharply defined, smooth and barely discernible, or coarse and rough.
The interspaces are the spaces contained within the network of lines or reticulations. They vary with the pattern of the reticulations, but generally they are best described as flat, convex or concave.The interspaces around the middle of the seed are more consistent in character than those at the ends which tend to vary more with the shape of the individual seed within classes or varieties.
The stippling is the small particles within the interspaces which may appear as dots or granulation.
Combinations of seed coat patterns are not always entirely consistent within classes or varieties but they do provide a very useful aid to identification. A binocular microscope is necessary to evaluate seed coat characteristics. The magnification used will depend on the individual preference but in some cases the identifying characteristics may be more easily recognized at low magnification.
Hilum or Seed Scar—The hilum or seed scar does not appear to be a very useful identifying character because of it variations within the same species. Some analysts attempt to use the nature of the hilum as an identifying characteristic but there is not always a consistent pattern.
Chalaza—A small raised mark near the hilum. This is also considered as a minor characteristic as no consistent value as an identification characteristic has been established. The chalaza has not been used as an identification tool in this manual.
Radicle—The primary root of the seed embryo between the pair of cotyledons. This appears in some classes as a ridge within a groove along the side of the seed. The combination of the groove and the ridge has some value as an identification characteristic. For example, it is evident in more seeds in samples of canola or rapeseed than in samples of wild mustard seed.