Plant do not have a digestive system like human.Why do they not need a digestive system?
Answers
Suppose you want to make a Lego house. You can make it with fresh Lego bricks, or may take another Lego house and then before making the new house, you have to first dismantle it.
If we compare a simple biomolecule (such as sugar, amino-acid etc) as a fresh Lego brick, and complex polymeric biomolecules (such as polysaccharide and protein etc. ); then plants obtain/ synthesize from inorganic minerals the lego bricks freshly, whereas animals first have to “dismantle” the larger biomolecules they have to intake. This “dismantling” process is called as digestion.
“Digestion” means a process of breaking the food-matter into smaller molecules.
Since plants are photosynthetic (prepare simple sugar from water and CO2), so they directly obtain simple sugars; and also the nitrogenous and other minerals they absorbs through root already exist in simple and soluble form; they does not need to “digest” the food the intake.
However, Just like animal cells, plant cells contain mechanisms to catabolize their own storage product (such as starch) and degradation of protein and other macromolecules. These could be compared to a sort of intracellular digestion.
As Brian Tremback correctly written- some plants those predate animals some of the food (mainly for nitrogen which is essential part of protein) , require a sort of breakdown or digestion process to utilize the prey. And that is well comparable to extracellular digestion at our stomach. But since most plants obtain the food directly as simple-molecules, they does not require a digestive system.