Question: Explain Variegation in Ivy (ii): shoot apical meristem

A group of cells (population) responsible of plant growth (above the ground) is called shoot apical meristem (SAM). These cells are remarkable in many ways. For example, they are able to differentiate as any cell type. They can end up as a mesophyll cell, epidermis cell, midvein cell, or any other plant cell. SAMs can be found at the growing tip of the plant, at the tip of every branch and in every node bud.

Botanists have named groups of cells. I will list them here without explaination. The outermost 2 layers of cells (for Ivy and many other species; some have only one layer) is called the tunica. The cell mass below the tunica is called the corpus. The layers are numbered too, beginning at the outside. L1 is the outermost layer of the tunica, L2 the second layer. L3 is equal to the corpus.

Cells of all layers contribute parts of a leaf when formed. L1 cells contribute the epidermis and, in some plants, tissues around the leaf margin. L2 cells contribute most of the mesophyll tissue. L3 cells contribute some of the mesophyll in the central region of the leaf near the midvein, the midvein and vasculature.
Source: Lecture 18: Shoot Apical Meristem (SAM) / Lecture 20: Leaves, Phil Becraft, 2005, http://public/iastate.edu/~bot512/lectures/SAM.htm / http://public/iastate.edu/~bot512/lectures/LEAVES.htm

This part of the exploration still needs some background knowledge to get it. This section, like the last one, refers to the building blocks of the leaf; in the next post I will get into that.

Question: Explain Variegation in Ivy

Answer:
Variegation in Ivy (and other plant species) is the occurrence of two or more colours on the leaves. Mostly this is caused by mutant chloroplasts which lost the ability to produce the green chlorophyll (called chimeric variegation) and the presence of colours other than green or the absence of colours. The patterns can be explained by the sectional way cells are contributed by the apical meristem layer cells when the leaf is developed.

Sources: The Gardener's Guide to Growing Ivies, Peter Q. Rose, 1996, Timber Press. Lecture 18: Shoot Apical Meristem (SAM), Phil Becraft, 2005, http://public/iastate.edu/~bot512/lectures/SAM.htm

As we look at the definition of variegation in Ivy you have to know a few things about the development of leaves to understand it. You also need to have some basic knowledge of the anatomy of a leaf. And thirdly the phenomenon chimera needs to be understood. In the next post I will briefly explain the development of leaves.