The secretory materials are those ergastic matters which do not serve as food, and so are not directly concerned with the nutrition and growth of the plants.
The three common secretory materials are: (1) Colouring Matters (2) Enzymes and (3) Nectar.
(a) Colouring Matters:
Plants possess different types of pigments which impart attractive colours to the parts. The green colour of the leaves and other aerial organs is due to the most essential pigment chlorophyll.
Photosynthesis, the all-important process of manufacture of carbohydrate food, is solely dependent on the presence of chlorophyll. It is a combination of two green pigments, chlorophyll a and chlorophyll b and the yellow pigments, carotin and xanthophyll.
The attractive colours, ranging from red to yellow, of the flowers and many fruits are due to carotenoids—carotin and xanthophyll. These are helpful in bringing about pollination, a necessary prerequisite to fertilisation, in case of flowers, and also in the dispersal of fruits.
The blue, violet and pink colorations are imparted by water-soluble pigments, anthocyanin, present in the cell-sap of the vacuoles. Even lower plants like algae, fungi and bacteria possess some pigments.
(b) Enzymes:
Enzymes are the ferments or digestive juices secreted by protoplasm. These are colloidal substances. Enzymes are the organic catalysts. They can catalise breaking down of complex food materials into their simpler soluble and diffusible forms.
There are different types of enzymes for the purpose, viz., diastase, for breaking down Starch ultimately into glucose; protease for nitrogenous matters; and lipase for fats and oils. Enzyme action is usually specific and often reversible.
Thus the enzymes operating on starch have no action on nitrogenous matters or fats and oils. Furthermore, the enzymes breaking down starch into sugar are equally responsible for the formation of starch out of sugar, showing thereby the reversible action of the enzyme.
Hundreds of enzymes are present in the plants. Apart from their role in digestion as stated above, they regulate or control all physiological processes involving chemical reactions. They influence the metabolic processes like photosynthesis, respiration and fermentation.
(c) Nectar:
This sweet substance is usually secreted by the floral parts of many plants. It is chiefly composed of sucrose together with glucose and fructose and is an excellent food. Nectar attracts a large number of insect visitors.
The latter in course of their search for nectar carry pollen grains to the stigmas of flowers and bring about pollination. It is thus used as a device for pollination in entomogamous (insect-pollinated) flowers.