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Left:  Sporangium of Rhizopus - at the top of the support stalk (sporangiophore) the tip swells up to form a spherical head. A prolongation of the stalk into the head forms a large swollen columella (blue)  The columella functions for nutrient exchange between the active protoplasm below and the developing spores inside the upper portion of the sporangial head (green).  The larger the head the larger the number of spores produced and the larger the columella  to allow sufficient nutrients to pass across.  Rhizopus has large sporangial heads and therefore needs a very large columella to supply the large mass of developing sporangiospores.  In this photomicrograph you will note that the columella is more than 2/3 rds the diameter of the head.  

Right: darkfield micrograph shows developing zygospore wih the two gametangia already cut off from the parent suspensors.  At the top left of  the micrograph you can see the edge of a   thick walled mature zygospore heavily protected by spiny outgrowths over its surface.

Students:

1. In Mucor the columella is only a third the diameter of the sporangial head or less. Explain!

2. How does protoplasmic streaming fit into this developmental equation?

3.  What is protoplasmic streaming? 

4. At what speed does protoplasm flow inside hyphae? 

5.  If protoplasmic flow is important for nutrient transfer and communication in non-septate hyphae how does it work in septate hyphae that have cross walls interrupting the flow.