I was thinking about plants having no digestion system (is that true at all?) and my conclusion was that they only ingest substances that they can use like that or that they can transform into usable ones (which then would maybe something similar to digestion).
So my question is:
Do plants "digest" substances that they ingested and then egest the "garbage"?
Also: can someone give me some hints where I can read about plant metabolism?
It's unclear to me if the question is asking generally about all plants, or only refers to digestion of food items by carnivorous plants.
If asking about plants generally, they don't to my knowledge perform any internal digestion. Non-carnivorous land plants take in materials in two ways, through their roots and through their leaves.
The roots take in water, along with molecules dissolved in the water. These include molecules containing nitrogen, phosphorous, and potassium, as well as other trace elements. Plants employ various strategies to enhance their uptake of water or nutrients, including pairing up with fungi or bacteria. Some also release low levels of acid to dissolve minerals for uptake. This could be considered a form of external digestion.
Leaves take in carbon dioxide. The CO2 is the primary source of carbon for the plant, and in turn is the main source of a growing plant's mass.
The waste that plants release is primarily O2, which exits the leaves through the same pores the CO2 enters. While roots release many types of molecules, these mainly have a functional purpose and aren't generally considered waste. Plants in salty environments also have mechanisms to expel salts.
So, no, plants in general do not perform internal digestion.
Now, carnivorous plants do capture and digest prey. This digestion is performed either by enzymes produced by the plant itself, or by microorganisms living in the environment of the trap. This would still probably be considered external digestion since it occurs either in open, liquid-filled pitchers, or on leaf surfaces that have folded over. The two sides of Venus fly trap traps do press together to form something of a seal while the prey is digested, but they open again when the meal is digested. The bladderworts, Utricularia spp., maybe have the best claim to "internal" digestion, with prey being dissolved inside small bladders covered by a trap-door mechanism. But these, too, originate from modified leaves or foliar structures.
Since the undigested parts of carnivorous plant meals are never really injested, they are not expelled.