Researching substances in laboratories is a modern way of trying to understand the universe. The various components of a subject are taken apart, isolated and carefully studied.
This is contrary to the old days, where people would observe a complete plant, animal or mineral in its natural surroundings, to gain information on its characteristics.
Nature-based cultures from all over the world believe in the interconnectedness of all things, as laid down in the wise saying “As above - so below”.

The basic idea is that every living thing has been blown in the same life force or spirit, while its physical form is influenced by a certain form of this universal energy.
The different forms of energy are symbolically represented by the stars and planets. Another way of saying is that a certain planet ‘rules’ several plants, animals and minerals.
Therefore a Mars-plant shares personality-treats with this solar body, and will be able to donate Mars-power as healing force. The Mars-plant can be recognized by its physical appearance and habitat.

A good example of a martial plant is the stinging nettle (Urtica dioica), a strong growing herb with a clear defense system.
The stinging nettle shares the strong life force and aggression of Mars. Therefore it is a useful herb for people who need more vitality, courage and assertivity. According to medical astrology Mars rules the blood, the stinging nettle has a reddish stem, and indeed this plant is a good bloodcleanser and bloodbuilder.
This form of cosmobiology is named the Doctrine of Signatures.
Modern chemistry has shown that this Doctrine is right, because the stinging nettle contains certain vitamines and a lot of minerals and other regenerative substances, what explains the energy supportive and cleansing qualities of the plant.

The Doctrine of Signatures states that the physical form of a plant gives a clue to its healing purposes.
Hereto one carefully observes the form, size and texture of the leaves, the stem, the roots, taste and smell of the plant, form and color of the flower, place of growing, and the way and time of flowering. The sum of these details forms the pattern or personality of the plant.
In herbology most of the time we work supportive and complimentary; so the aggressive stinging nettle is a good choice to balance a too soft character, but I would never give it to a fiery and irritable personality.

Here are the basics used in the Doctrine of Signatures:

The roots of the plant have their influence on the brains of a human being; plants with impressive roots are good to ground airy people.
The stem and leaves represent the middle part in a human being; think of the breast, heart and lungs. Further the stem is analogous to our skeleton.
The flowers mostly work on the lower belly and the genitals.

Upward growing plants fully open up to the force of the sun, and therefore bring optimism and enthusiasm.
Downward pointing plants with hanging flowers harmonize people who feel down or depressed; so here Like-cures-like.
Horizontally growing plants symbolize contact and exchange and therewith promote the circulation of blood and lungs.

A round stem is associated with femininity, harmony and softness.
A square stem points at the masculine qualities of immunity and strength.
A hollow stem reveals that the plant may relief complaints with the hollow organs in a human being, like the lungs or the gullet.
A hairy stem and dito leaves show action on the skin, hair and mucous membranes.
Thorns and stings show that the plant promotes regeneration and immunity.

Small and divided leaves show a contracting force in the plant, but have a relaxing effect on a human being.
Large leaves on the other hand have contracting power in a human being and may help stop profuse bleedings or increase muscle contractions.
Dark-green leaves work calming and vitalizing.
Yellow-green leaves promote the metabolism.

Dark yellow flowers stimulate the life force and the liver
Light yellow flowers are good for internal cleansing of the body.
Red flowers work on the heart, bloodcirculation, energy and vitality.
Orange flowers work on the solar plexus and bring enthusiasm.
Pink flowers work softening and harmonizing.
Blue flowers cool down heat complaints like feaver, inflammations or an irritable character.
Purple flowers Relax the nervous system and make a person more introvert.
White flowers work soothing in inflammations and have their special working area on the female organs.
The color brown brings inner calm and strength, especially in case of exhaustion.

Sun-plants have long strong stems and upward flowers in the colors white, yellow or orange.
Moon-plants are watery and have large leaves.
Mercury-plants grow horizontally and easily move along with the wind.
Venus-plants are soft-colored and smell deliciously.
Mars-plants have thorns and stings.
Jupiter-plants are big and grow abundant, like in the jungle.
Saturnus-plants are the strong ones that survive scarce conditions, like cactae.
Uranus-plants are the radio-active ones.
Neptune-plants are poisonous in the bad case, or have purple flowers and bring relaxation to the mind in the positive case. Can’t help it: Neptune is both for the very high and the very low.
The plants of Pluto are the toad-stools and mushrooms.

This is not a system one learns overnight, and to make use of it one also needs his intuition.
I believe that, if we truly wish to understand the healing riches of the vegetable world, it is most wise to combine the traditional knowledge of the Doctrine of Signatures with the scientifically derived conclusions of chemical tests with the plants.