The way plants grow and develop is fascinating and distinct from the way animals grow and develop. In this blog post I want to give plant hobbyists a deeper appreciation of the precise nature of how their plants grow and a base of knowledge to better comprehend phenomenon like variegation and cresting which will be explored in future blog posts.
Part 1: the Meristem
Imagine if on the top of your head there was a dome of undifferentiated embryonic tissue. This tissue would constantly make new organs-new arms would be produced out of this embryonic dome and your old arms would shrivel away and fall off. On the other side of your torso there was another dome of embryonic tissue, only this one produces new legs. Your body would have other embryonic domes that produced your gonads-of which you would have multiple! Sounds like a science fiction story about life on another planet. But this story is about life here on earth. This is the story of how plants grow. At their growing tips plants have tiny domes of undifferentiated cells called meristems. There are different kinds of meristems-Shoot Apical Meristems (SAMs) that produce leaves and stems, root meristems that produce roots and floral meristems that produce flowers, intercalary meristems at leaf-stem junctions and lateral meristems involved in growth of tree trunks. Meristems are too small to be seen by the naked eye but can be visualized under the microscope.
Want to see a merisitem up close? Let's have a look. The images bellow are taken from a review a paper published in Critical Reviews in Plant Sciences. A link to the publication is here.
These are microscopic images of the growing tips of a Barley plant. The image on the left is a scanning electron micrograph of the shoot tip. Look at the dome shaped Shoot Apical Meristem (SAM) in the middle. The cells in the meristem are in an un-differentiated or embryonic state. These cells continuously grow and replenish the meristem while giving rise to plant organs (leaves, roots, flowers etc.). Look at the rings of tissue surrounding the meristem (labeled P1 and P2. "P" stands for Primordia). These are baby leaves, known as leaf primordia.
The image on the right is a cross section of a shoot showing the SAM and leaf primordia.
The Shoot Apical Meristem (SAM) with baby leaves (P1 and P2)
The meristem is comprised of different layers that give rise to different parts of the plant. The outer layer gives rise to the epidermal layers, while the inner layers give rise to the inner leaf tissues.
The images bellow are taken from a review a paper published in Current Opinion in Genetics & Development and are images of shoot and root apical meristems. In the shoot tip on the left, we can see the layered structure of the shoot meristem. The outermost layer is highlighted in red, the second
in yellow. Blue marks the slowly dividing central zone that replenishes the meristem. On the right is a root tip where the root mesmerism highlighted in green and blue.
What about plants that are leafless like cacti? Do Cacti Shoot Apical Meristems produce leaf primoridia even though cacti are typically leaf-less? The answer is that apparently they do, but the leaves remain microscopic. The image below is taken from a comprehensive scientific study on cactus meristems. A link to the study is here.
Cross section of a shoot apical meristem from the cactus Lobivia ferox
Meristems lay down the blueprint for a plant's growth, development and appearance. They tie into several phenomenon of interest to plant people. In my next blog post I will explore how the meristem ties into phenomenon of interest to succulent aficionados: variegation and cresting.