I believe it is nervous tissue
I have searched everywhere, but I have not found the proposals of the question, but I will explain to you what is the endoplasmic reticulum so that you can answer it.
The endoplasmic reticulum is a eukaryotic organelle located in the cytoplasm.
The endoplasmic reticulum is a network of membrane tubules (often interconnected) scattered throughout the cytoplasm of eukaryotic cells. Its membrane, which alone represents more than half of the cellular membrane system, is in contact with the nuclear envelope.
The endoplasmic reticulum can be:
Granular (or rough) (RER) that is to say associated with ribosomes.
Smooth (SER).
The granular endoplasmic reticulum is the place of synthesis (in the associated ribosomes) of the proteins secreted outside the cell and of the proteins and lipids constituting the membranes of the cellular organelles. Golgi, lysosomes, mitochondria, nucleus, ribosomes, vesicles ...). It participates in the correct folding of the proteins that have just been synthesized.
The smooth endoplasmic reticulum participates in cellular metabolism, synthesizing lipids and storing calcium.
This is just an educated guess. As the purple pigment, caused by anthocyanins, are on the bottom side of the plant, their purpose could probably be to direct the light.
Specifically, a purple pigment on the bottom of the plant would absorb certain wavelengths near the purple spectrum, and reflect other wavelengths back toward the portion of the plant with chlorophyll. In this way, light does not simply pass through the leaf, instead it is reflected back in towards the chlorophyll to maximize the amount of light being used for photosynthesis.
