Final answer:
In a flax stem, parenchyma cells make up both the central pith and the outer cortex. Sclerenchyma cells, which are used to make linen and rope, are found as fibers within the flax plant and provide structural support.
Step-by-step explanation:
The question refers to the sclerenchyma cells which are present in strands such as those found in linen flax. In the flax stem, the parenchyma cells make up the central pith and the outer cortex. These cells are important for storage and photosynthesis within the plant. However, the sclerenchyma cells are known for providing structural support and are usually dead at maturity. There are two types of sclerenchyma cells: fibers, which are long and slender and used to make linen and rope, and sclereids, which are smaller and can affect the texture of fruits like pears. The correct answer to which layers of the stem are made of parenchyma cells is 'a. cortex and pith' based on the information provided.