Abstract Detail

Paleobotany

DAntonio, Michael [1], Boyce, C. Kevin [2].

Potential secondary phloem in Stigmaria and long-distance sugar transport in arborescent lycopsids.

Extinct arborescent lycopsids produced secondary xylem for hydraulic transport but have not been thought to have produced secondary phloem for photosynthate transport. However, phloem is a delicate tissue that rarely survives diagenesis; even primary phloem often cannot be positively documented in fossil plants and must be generally assumed to have been present. A lack of secondary phloem has been a particular challenge for understanding the physiology of potentially long-lived lycopsid trees, leaving different researchers to assume either the persistence of functional primary phloem despite potential disruption from wood growth or that the plants somehow survived without long-distance sugar transport. Here, we describe potential secondary phloem from an extraordinarily well-preserved fossil of Stigmaria, the arborescent lycopsid rooting system. The tissue immediately adjacent to the wood cylinder can itself be recognized as secondary because the rays continue out from the wood and transit the full thickness of this wood-adjacent tissue. This tissue is composed of polygonal to ellipsoidal cells with lumen diameters 15-65 μm and cell wall thicknesses ~5 μm. A few potential sieve cells have been preserved in transverse section, exhibiting what are interpreted as sieve plates with scalariform sieve areas averaging 6.1 μm wide. The individual features within the scalariform sieve plate average 2.4 μm wide. These dimensions are distinct from those of the wood tracheids in this fossil and their wall thickenings and compare favorably to those of living relatives for which phloem can be observed. This discovery would establish a mechanism for continued physiological viability of arborescent lycopsids despite the anatomical disruption caused by wood growth and demonstrates how much we can still hope to learn from fossil organisms—even those that have been intensively studied for 200 years.

1 - Stanford University, Geological Sciences, 450 Jane Stanford Way, Building 320, Stanford, CA, 94305, United States
2 - Stanford University, Geological Sciences, 450 Jane Stanford Way, Bldg. 320, Stanford, CA, 94305, United States

Keywords:
Lepidodendron
Sigillaria
Phloem
Sugar transport
plant physiology.

Presentation Type: Oral Paper
Session: PAL1, Cookson Award Session I
Location: Virtual/Virtual
Date: Monday, July 27th, 2020
Time: 10:00 AM
Number: PAL1001
Abstract ID:358
Candidate for Awards:Isabel Cookson Award,Maynard F. Moseley Award