Meristems-1    
311 Syllabus

There are two kinds of Apical Meristems.

The Primary Apical Meristems are formed during Embryo Development. 

Branching of the Shoot and Root involves the creation of Secondary Apical Meristems.

The term Secondary Root applies to Branch Roots.

This terminology is not commonly used for the Shoot System.

However, Stem Branches contain Secondary Apical Meristems and these could be called Secondary Shoots.

The Root and Shoot Apical Meristems become defined
during Embryogenesis.  Secondary Meristems which give rise to branches arise later in development.

The Apical Meristems of Seed Plants are Multicellular.

They may function over many years or even centuries in the case of certain gymnosperms like bristle-cone pine and giant Sequoia.

They are said to have Indeterminate Growth!

Meristems may contain Initials. These are undifferentiated cells that undergo mitosis at a slow rate. Their  derivative cells continue to divide, but eventually differentiate into one of the three Primary Tissues.

Root Development

The Root is very conservative in terms of its evolution.

It is designed to penetrate the soil.

Consequently, branching does not occur at the tip, but they arise basipetally (towards the base). Their origin is Internal. Both of these factors prevent damage to young branch roots.

The Root Apical Meristem is covered by a Root Cap which physically protects it.

The Root Cap also secretes mucilaginous carbohydrates which act as a Lubricant.

Aerial Roots also have Root Caps. These protect the RAM from desiccation. Pandanus and Coconut Palm are two local examples of this.

Most roots produce Root Hairs. These occupy a zone that is posterior to the elongation (growth) zone. This helps to protect them from physical trauma.

They greatly increase the absorptive surface area of the root.

They Secrete Chemicals into the Rhizosphere (immediate area around roots) which have ecological significance for microbes in the soil and for the roots themselves.