Final answer:
Matthias Schleiden and Theodor Schwann researched the form and structure of plant tissue and single-celled organisms called 'animalcules.'
Step-by-step explanation:
The subject of this question is Biology. The question is asking about who researched the form and structure of plant tissue and single-celled organisms that were called 'animalcules.'
The research on plant tissue and single-celled organisms was conducted by two scientists named Matthias Schleiden and Theodor Schwann. Matthias Schleiden was a German botanist who studied plant tissues and described them as being composed of cells.
Theodor Schwann, a German physiologist, made similar microscopic observations of animal tissue and realized the similarities between plant and animal tissues, which laid the foundation for the idea that cells are the fundamental components of plants and animals.