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Name: Jules
Status: student
Grade: other
Location: Outside U.S.
Country: France
Date: Spring 2011


Question:
I read the question about pine cones wet and dry. My question is : what is the structural mechanism in detail, that is at the origin of opening and closing ?



Replies:
Pine cones are comprised of two scale types – seed scales (often called ovuliferous scales since they act as a “womb” for the developing seed) and bract scales. The seed scales are the most conspicuous. Bract scales are reduced in size and subtend (meaning they accompany) the seed scales.

Seed scales can actually detect and adapt to changes in relative humidity. Wetness causes the scales to stiffen up and close in sync. When conditions are dry, they reopen.

How does this process work mechanically? The inner surface of the seed scale has elongated sclerenchyma fiber (strength and support) cells, while the outer surface has sclerids (short sclerenchyma cells). Although both of these cell types are technically dead, they allow for passive uptake and release of water. Both cell types have cellulose microfibrils (like threads) that wind around the cells. The winding angle determines how the cell reacts when absorbing water. A high winding angle (sclerids) allows elongation when damp. A low winding angle (fibers) resists elongation. When the two work in concert, scales will tighten or loosen up based on the relative humidity, allowing the whole cone to close or open.

Incidentally, this phenomena has been exploited to create “impossible bottles” – allowing seemingly oversize cones to be placed in a narrow - Hide quoted text - necked bottle. The reality is that the cones were exposed to damp conditions, placed in the bottle, and then allowed to dry and expand.

Dr. Tim Durham Instructor, Office of Curriculum and Instruction University Colloquium Department of Biological Sciences



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