```python
class NoSuchStudentException(Exception):
    pass


class Garden:
    DEFAULT_STUDENTS = [
        "Alena", "Boris", "Cyril", "Daniel",
        "Eva", "František", "Gitka", "Helena",
        "Ivana", "Jozef", "Karol", "Ladislav"
    ]

    PLANT_MAP = {
        'G': 'Tráva',
        'C': 'Ďatelina',
        'R': 'Reďkovky',
        'V': 'Fialky'
    }

    def __init__(self, diagram, students=None):
        if students is None:
            self.students = self.DEFAULT_STUDENTS.copy()
        else:
            self.students = sorted(students)

        self.r = diagram.splitlines()

    def plants(self, student):
        if student not in self.students:
            raise NoSuchStudentException(
                f"Student '{student}' is not assigned a plot."
            )

        i = self.students.index(student)
        c1 = i * 2
        c2 = i * 2 + 1

        student_plants = [
            self.r[0][c1],
            self.r[0][c2],
            self.r[1][c1],
            self.r[1][c2]
        ]

        return [self.PLANT_MAP[code] for code in student_plants]
```