The species, which I want to show you today is Galeopsis tetrahit L. from the Lamiaceae family. In German, it's known as “Gemeiner Hohlzahn” and in English, it's called “Common hemp nettle”.
G. tetrahit - habitus
It's a small, annual plant, which can reach heights between 10 and 30 centimetres (sometimes 50 centimetres) which corresponds to 3.9 ton 11.8 (19.9) inches. The stalk grows upright and much thickened at the nodes. The nodes are covered with many bristles.
G. tetrahit - thickened node with bristles
The leaves are lanceolate to egg-shaped an follow the decussate leaf-pattern. The have a strong serrated or lobed margin and are bald. The also have a short petiole.
G. tetrahit - leaf
The inflorescences are whorls, which are located in the axillary of the upper leaves. The petals are green and covered with bristles. They are also very pointed. So, the Calyx of G. tetrahit is very pungent (s. pictures). The petals form the typical, cygomorphic “lip” of the Lamiaceae and have a pink colour but can also be purple or even white. The centre is yellow.
G. tetrahit - calyx
Flowering time is between June and October. The primary pollinators are bees or bumblebees. two special humps on the lower lip cause the head of the insect to the nectar. These two humps, which looks like teeth, are typical for the Genus Galeopsis and also the reason for the German Name “Hohlzahn” (what means “hollow tooth”).
G. tetrahit - flowers with humbs (arrow)
The ripe fruits are the typical chiusa (“Klausen” in German), which are unique for the Lamiaceae and the Boraginaceae. Animals remain at the spiny calyx and hurl the fruit when they rid themselves.
G. tetrahit is a very common plant in Europa but can also be found as neophyte in North America. It grows on dry and stony places and prefers nutrient rich soils (it's also a indicator for nitrogen). It can be found on ruderal wastelands, slopes and at roadsides and railroad tracks. Originally, the species is a natural interbreed between two other plant of the Genus Galeopsis (Galeopsis pubescens Bess. Fries and Galeopsis speciosa Mill.