Equistetum sp. sporangium wall
Transverse section of a Equistetum sp. strobili (spore producing structure). The strobili as found is shown at: www.flickr.com/photos/14643312@N02/18511410355/. Equistetum or Horsetail is a vascular plant which produces spores instead of seeds. The specimen is from the edge of the Dowagiac River.
This is the sporangium wall under crossed polarizers with a Lambda wave plate. In the main image, the inside of the sporangium is to the top. In this optical configuration, optically isotropic material is purple and birefringent material varies from orange to blue depending on crystal orientation. The inset image is rotated 45 deg. relative to the main image with constant polarizer and analyzer orientation. This shows that the optically active material forms a spiral around the outer sporangium cell walls. Differential focusing also supports this conclusion.
There are several sources of optically active material in botanical structures with common examples being calcium oxalate, silica, and ordered cellulose structures, the later commonly making up the Xylem walls. Equistetum stem structures are known to contain silica in the epidermal walls, with an example at: www.flickr.com/photos/14643312@N02/17227273543/in/album-7.... While I cannot conclusively say what this structure is, I speculate that it is ordered cellulose. Could it be that this structure is to facilitate spore release? I doubt this as the spore diameter is about 2X the outer sporangium cell wall diameter. Another possibility is that as the strobilus drys, the sporangium cell walls would better retain snap such that the spores could be released through the decaying sporangiophore, which shows less optical activity. So, I need to do some research in the literature.
The protocol was as follows. Specimens fixed in FAA (formaldehyde, acetic acid, ethanol) 24 hr. Dehydrated in 35, 50, 75, 85, 95, 99 % IPA in water, 6 hours each min. Infiltrated in xylene saturated with Paraplast for 2 days, followed by 2 changes of melted Paraplast for 2 hours each. Embedded in Paraplast. Sectioned on a Spencer 820 microtome at 11 micron. Cleared in Xylene 2X, 10 min each. Rehydrated 99 (10 min), 90 (10 min.), 85, 70 % IPA, 2 min. each. Stained Gill's Hematoxylin 20 sec. Washed 3 min running water. Blued 0.05 % lithium carbonate 3 s. Water rinse 1 min. Stained 1 % aq. Erythrosin-B 2 min. Dehydrated 99 % IPA 2 min 2X. Cleared 2X xyene 5 min each. Mounted with DEPEX.
Photographed on a Spencer 42 petrographic polarizing microscope using an original magnification of 430X, using a Sony NEX 5N with a Leica MIKAS 1/3X adapter.
Equistetum sp. sporangium wall
Transverse section of a Equistetum sp. strobili (spore producing structure). The strobili as found is shown at: www.flickr.com/photos/14643312@N02/18511410355/. Equistetum or Horsetail is a vascular plant which produces spores instead of seeds. The specimen is from the edge of the Dowagiac River.
This is the sporangium wall under crossed polarizers with a Lambda wave plate. In the main image, the inside of the sporangium is to the top. In this optical configuration, optically isotropic material is purple and birefringent material varies from orange to blue depending on crystal orientation. The inset image is rotated 45 deg. relative to the main image with constant polarizer and analyzer orientation. This shows that the optically active material forms a spiral around the outer sporangium cell walls. Differential focusing also supports this conclusion.
There are several sources of optically active material in botanical structures with common examples being calcium oxalate, silica, and ordered cellulose structures, the later commonly making up the Xylem walls. Equistetum stem structures are known to contain silica in the epidermal walls, with an example at: www.flickr.com/photos/14643312@N02/17227273543/in/album-7.... While I cannot conclusively say what this structure is, I speculate that it is ordered cellulose. Could it be that this structure is to facilitate spore release? I doubt this as the spore diameter is about 2X the outer sporangium cell wall diameter. Another possibility is that as the strobilus drys, the sporangium cell walls would better retain snap such that the spores could be released through the decaying sporangiophore, which shows less optical activity. So, I need to do some research in the literature.
The protocol was as follows. Specimens fixed in FAA (formaldehyde, acetic acid, ethanol) 24 hr. Dehydrated in 35, 50, 75, 85, 95, 99 % IPA in water, 6 hours each min. Infiltrated in xylene saturated with Paraplast for 2 days, followed by 2 changes of melted Paraplast for 2 hours each. Embedded in Paraplast. Sectioned on a Spencer 820 microtome at 11 micron. Cleared in Xylene 2X, 10 min each. Rehydrated 99 (10 min), 90 (10 min.), 85, 70 % IPA, 2 min. each. Stained Gill's Hematoxylin 20 sec. Washed 3 min running water. Blued 0.05 % lithium carbonate 3 s. Water rinse 1 min. Stained 1 % aq. Erythrosin-B 2 min. Dehydrated 99 % IPA 2 min 2X. Cleared 2X xyene 5 min each. Mounted with DEPEX.
Photographed on a Spencer 42 petrographic polarizing microscope using an original magnification of 430X, using a Sony NEX 5N with a Leica MIKAS 1/3X adapter.