- In the first two pictures (after the boy’s fingers image) is a Poaceae sp. In this case the mechanical lesions are more frequently because leaf blades of many grasses are hardened with silica phytoliths being able to cut human skin.

- More irritant is Urtica dioica (third picture): the leaves and stems bear many stinging hairs (trichomes), whose tips come off when touched, transforming the hair into a needle that will inject several chemicals: acetylcholine, histamine, 5-HT (serotonin), moroidin,[3] leukotrienes,[3] and possibly formic acid.[4][5] This mixture of chemical compounds causes a painful sting or paresthesia  (https://en.wikipedia.org/wiki/Urtica_dioica)

- In the next picture is a dried inflorescence of clover species (I suppose red clover – Trifolium pratense) a medicinal plant. Even dietary amounts of red clover are safe, the medicinal quantities may cause rash-like reactions, muscle ache, headache, nausea etc (https://en.wikipedia.org/wiki/Trifolium_pratense)

- As you can surmise, in the fifth picture is Lamium maculatum, nontoxic plant.

- The last photo contains Sambucus nigra inflorescence; a lot of part of  this species (flowers, fruit, bark) have  large medicinal uses.

I hope these information are useful.

I think it's plant from genus Thalictrum (Ranunculaceae), but i don't know species is it

Marko, rastline ne poznam. Sem pa sliko poslala prijatelju biologu, izolanu Bojanu Kocjančiču. Zdaj čakam odgovor...

LP Irena K.

Even if the picture doesn’t have sufficient pixels, this plant looks very much like lovage (Levisticum officinale), food and aromatic species. Its roots contain coumarin that can causes photodermatoses and photosensitivity in people allergic or abnormal skin phenomena due to sunlight.

Apiaceae - Levisticum officinale

I Think only urtica may cause such skin eruption 

 Are all the pictures from a single plant?.....this will give a clue to answering your question

- In the first two pictures (after the boy’s fingers image) is a Poaceae sp. In this case the mechanical lesions are more frequently because leaf blades of many grasses are hardened with silica phytoliths being able to cut human skin.

- More irritant is Urtica dioica (third picture): the leaves and stems bear many stinging hairs (trichomes), whose tips come off when touched, transforming the hair into a needle that will inject several chemicals: acetylcholine, histamine, 5-HT (serotonin), moroidin,[3] leukotrienes,[3] and possibly formic acid.[4][5] This mixture of chemical compounds causes a painful sting or paresthesia  (https://en.wikipedia.org/wiki/Urtica_dioica)

- In the next picture is a dried inflorescence of clover species (I suppose red clover – Trifolium pratense) a medicinal plant. Even dietary amounts of red clover are safe, the medicinal quantities may cause rash-like reactions, muscle ache, headache, nausea etc (https://en.wikipedia.org/wiki/Trifolium_pratense)

- As you can surmise, in the fifth picture is Lamium maculatum, nontoxic plant.

- The last photo contains Sambucus nigra inflorescence; a lot of part of  this species (flowers, fruit, bark) have  large medicinal uses.

I hope these information are useful.

The photos with Poaceae species are: Sorghum halepense inflorescence (first photo after the boy's fimgers image), stems and leaves from Agropyron sp. I think (second foto). About the fourth photo it seems to be Allium sp. inflorescence, not Trifolium pratense (my opinion, it doesn't look like dried Trifolium inflorescence). About the other species I agree with Gabriela Voichita opinion.

Thanks all for helping me to identify the plants and find out which one caused the patient's problem. I agree with Gabriela P. that the fourth photo was an Allium sp. (not Trifolium) and disagree with her about the Poaceae photos. There were not parts of the same plant. Of course it is impossible to be sure from the photos.

I surmise that the boy's rash was an allergic contact dermatitis, because it appeared the day after the contact and was getting worse during the first week. It could hardly be a toxic reaction. I expected that it would be caused by sesquiterpene lactones, that according to the literature, are the most frequent cause of such dyshidrosiforme (pompholyx) hand eruptions. So Levisticum officinale (or any other Apiacea the plant could be) should be the most probable culprit of this skin rash.

If in future he will still experience troubles with plants, we will patch test him with standard haptens and fresh plants, although it is far from simple to prove a possible contact allergy.  

Any further idea about the plants and their biologically active constituents?

I believe that there is no plant that could cause this type of burn. These plants (Thalictrum, Calamagrostis sp.?, Lamium maculatum, Allium, Sambucus), as far as I know, there is no ability to cause burns to the skin surface. Perhaps this other plants, such as Heracleum, Trifolium, Hypericum, Urtica. It is also possible, that the specific allergy.

In my opinion, I see a Thalictrum species rather than Levisticum officinale. I don't know whether Thalictrum, likewise other Ranunculaceae, could produce such kind of skin rash.

About the other species, I would exclude them from the list of potential culprits.

Anyway, plants other than those in the pictures (e.g., Apiaceae, Rutaceae, Hypericaceae, Ranunculaceae) could also have been caused this problem, even as simply brushed during the walk.