Leaves- Leaf Initiation - Dicots

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The following account is based on the analysis of Dicots. I will deal with monocots like sugarcane later. Leaf Initiation normally starts with a Periclinal division in a subepidermal cell(s). However, divisions in the Epidermis follow in short order. Epidermal divisions are all Anticlinal while divisions in the underlying area occur in many planes. This is similar to what we saw in the SAM.  This results in a hemispheric bulge that is located just below the Peripheral Zone of the Apical Meristem. Careful analysis has shown that these Leaf Primordia arise with geometric precision. Subsequent growth of the leaf is decentralized.

RicinSAM240Lab.jpg (25060 bytes) ColeusEarlyLeafPrim240.jpg (38317 bytes) ColeusLateLeafPrimLab.jpg (91591 bytes)

Dicot Leaf Initiation

1] Initially, the Dicot Leaf Primordium is dome-shaped. DicotSAMLfPrim3-3D.jpg (34067 bytes)

2] However, it starts to elongate and becomes cylinder-like.

3] It becomes dorsiventral because the side facing the apical meristem is  flattened due to cell enlargement.

I have tried to show these stages in the diagram on the right.
MonoLp-2-3D.jpg (25767 bytes)
Monocot Leaf Initiation-1		 MonoLp-1-3D.jpg (27684 bytes)
Monocot Leaf Initiation-2		 MonoLp-3-3D.jpg (33046 bytes)
Monocot Leaf Initiation-3
MonoLp-4-3D.jpg (35528 bytes)
Monocot Leaf Initiation-4
MonoLefIni-2-3D.jpg (38003 bytes)
Monocot Leaf Initiation-5
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Many Monocots produce leaves that ensheath the stem at the node.

1] Their Leaf Primordia are ridge-like.

2] Growth extends laterally from the point of initiation and eventually can form a complete cylinder.

3] Apical growth is very limited as an intercalary meristem develops and produces the rest of the leaf.

Subsequent aspects leaf development are complex. I will try to explain the main features.

Dicot Leaf Development

In most cases apical growth is minimal. The entire margin of the dorsiventral leaf primordium can be meristematic. These observations gave rise to the concept of Marginal & Submarginal Meristems in leaves.

The classic interpretation was  that  superficial cells at the leaf margin divided anticlinally to produce the entire Epidermis (Upper & Lower). This was called the Marginal Meristem.

Internal cells at the leaf margin divided anticlinally and periclinally to produce the internal tissues (Ground & Vascular). This was called the Submarginal Meristem.

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