Tyloses and ecophysiology of the early carboniferous progymnosperm tree Protopitys buchiana

Abstract

Trunk woods of Early Carboniferous Protopitys buchiana show the earliest example of tylose formation and the first record for a progymnosperm. Protopitys tyloses are more densely located in inner trunk woods and near growth layer boundaries. We suggest, therefore, that an altered physiological state of living ray cells, during dormancy and/or following water stress, was necessary to make the woods vulnerable to tylose formation. Coupled with the distribution and proximity of abundant wood ray parenchyma to large xylem conducting cells, the positions of conduits filled with tyloses can be interpreted as ecophysiological responses of the plant to changes in local environment. In addition, some xylem conducting cells might have functioned as vessels. Fungal hyphae are present in some tracheary cells and in some areas with tyloses, but there is no evidence for wood trauma; we conclude, therefore, that these particular cases of tyloses are probably not induced by wound trauma. Protopitys buchiana wood thus shows structure/function similarities to modern woods with vessels, such as those of dicot angiosperms. This implies that ancient and modern plant ecophysiological responses correlate well with the physical parameters of their cellular construction.

Fig. 1. Transverse sections of Protopitys buchiana wood (BOU 1500 AT 01). Trunk exterior is up in A–C. A, Outer growth layer of a trunk that shows two closely spaced growth pauses (arrows). Ray parenchyma and tyloses are brown‐coloured cells. B, Inner growth layer (arrows) of the same trunk. C, Enlarged area near that shown in A, showing tyloses (T) inside tracheary cells, and the physical relation between rays (R) and tracheary cells with tyloses. Bars = 200 µm (A and B) and 100 µm (C).

Fig. 2. Radial longitudinal sections of Protopitys buchiana wood (BOU 1500 BLR 01). A–D, Views of xylem rays (R) and tracheary cells with tyloses (T). A, Tyloses present in overlap areas of some rays as they pass over mid‐regions of larger tracheary cells. Possible vessel perforations at arrow. B, Well‐preserved area showing cellular/pseudocellular tracheary cell‐filling by tyloses (T), scalariform‐bordered pitting, and ray cross‐field pitting (R). C, Enlarged area showing tyloses (T) inside tracheary cells and ray cross‐field pitting (R). D, Higher magnification of A to show possible vessel perforations by loss of pit membranes (lower arrows). Most other pits have intact pit membranes (e.g. upper arrows). Ray cross‐field pitting (R) visible at top. Empty cell lumens (L) contrast with those filled with tyloses (T). Bars = 100 µm.

Fig. 3. Tangential longitudinal sections of P. buchiana wood. A, View of xylem rays (R) and a group of four cellular/pseudocellular tyloses (T and four cells beneath it) generated by protrusion of ray parenchyma of one ray (arrow) into an adjacent tracheary cell (to left of ray cell at arrow). Sample 491, Solms‐Laubach 1897. B, View of undulate tracheary cells and density of ray contacts. BOU 1501 BLT 01. Bars = 100 µm.