Q. Show the Directly Visible Chemical Characters?
Very few chemical substances in plants can be observed directly, but the few substances such, starch grains which are present in most green plants as food reserves have been used for chemotaxonomic purposes. The type of starch grains present in different plants are very specific and this information can be used without any ambiguity. Reichert (1913) examined the starch grains in 350 species of plants and established their differentiation and specificity of occurrence in relation to genera and species, thus providing chemotaxonomic information. Tateoka (1962) reviewed the starch grain form in the grass family (Gramineae) and used this as additional information to classify the finally into tribes. For example, in the Tribe Hordeae, the typical members such as Hordeum have compound starch grains, while other genera like Lolium, Nardus, and Papapholis have simple starch grains.
Raphides are the crystals of calcium oxalate which are present in large cells in different plant tissues and can be observed directly. They are long needle shaped crystals, pointed at both ends and usually occur in bundles, thus being easily identified. They have been observed in as many as 35 families of angiosperms. Several families of the Order Centrospermae and the Family Cactaceae show the presence of raphides. This feature along with other chemical characters and the similarity in embryological characters strengthens the suggestion that the Family Cactaceae shows some relationship with the Order Centrospermae.