Mature cells with specific roles, divided into Simple (Parenchyma, Collenchyma, Sclerenchyma) and Complex (Xylem for water and Phloem for food). 2. Anatomy of Vegetative Organs This is usually the "meat" of any PDF on the subject:
A free, peer-reviewed textbook from Rice University that covers plant structure in high detail. Download ANATOMY FLOWERING PLANTS pdf
If you are looking for academic-grade texts, I recommend searching for these specific open-access sources: Mature cells with specific roles, divided into Simple