1371N.HTML????????// Leaf Abscission

 

Vascular system broken by mechanical means

Protective layers of cells are formed

Nearly complete abscission in ash (Fraxinus sp.) as shown in a longitudinal section of the petiole. Note how the vascular system is still largely intact. It is typically broken by mechanical stress (e.g. wind, contact with animals, etc.).

The abscission zone is subdivided into a separation layer (arrow) of thin-walled empty cells through which the actual break occurs, and two protective layers at both sides of the separation layer. Protective layers appear dark brown due to the deposition of specific "protective" substances (e.g. suberin) in their cells.

The protective layer that forms on the stem side serves to seal off the exposed stem tissues in order to protect them from water loss and from the invasion of microorganisms. In this regard, the protective layers are similar to periderm. Abscission of the leaf will leave a leaf scar appearing on the side of the stem.

 
   
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