BORDERED PITS in RAY CELLS
BORDERED PITS IN RAY CELLS [ PDF ]
In 1981, in a paper on wood anatomy of Chloanthaceae (=Dicrastylidaceae), I published a photograph showing bordered pits on ray cells. Living ray cells--we’re not talking about some funny kind of vessel or tracheid, as in the ray tracheids of conifers. We’re not talking about perforated ray cells. In fact, in no fewer than 15 other papers and in my book Comparative Wood Anatomy, I published pictures of bordered pits in living ray cells. Nobody was noticing this feature. It wasn’t in glossaries. My reports were never cited. So I did a survey of this phenomenon and discovered it to be widespread in wood of angiosperms and Gnetales. The explanation for this phenomenon was functional, of course.
Ray cells contribute to the strength of wood---otherwise all of that cellulose and lignin in ray cell walls would be a complete waste, and plants don’t waste. Bordered pits are a compromise device: maximizing wall strength while maximizing conduction from one cell to another. Bordered pits are abundant on tangential walls of ray cells, indicating that conduction of photosynthate-containing fluid is active. There are also some, but fewer, bordered pits on other walls of ray cells, as well as simple pits intermixed with bordered pits. And axial parenchyma shows pitting conditions similar to those of ray cells. My survey showed that these phenomena are very common---thousands of woods have it. Extremely few drawings of wood show bordered pits on ray cells—those who draw woods seem to assume that ray cells pits are all simple, and draw them that way. Microcasting, a recent technique, does show it (but those who do microcasting of woods look at vessels and tracheids and almost never at rays). I submitted the paper to Botanical Journal of the Linnean Society. The sole reviewer said that either I had the definition of bordered pits wrong, or my observations were wrong. In other words, in order to conform with the incomplete observations of earlier wood anatomists, I should deny my findings. Sorry, findings are findings! The bordered pits on ray cells may be small, but they are still bordered. Simple pits can be present in addition to bordered pits. My observations didn’t attempt to list the taxa in which bordered pits do occur on ray cells, because literally thousands of dicot woods would have to be listed (I am guessing that bordered pits can be found on ray cells in wood of about 100,000 species of dicotyledons) The real message of the study on bordered pits in ray cells was to show that ray cells have functions, and that functions are related to structure. Physiological wood anatomists have focused almost exclusively on the activities of tracheary elements and neglected how the parenchyma systems in wood work. Textbooks describe rays, but don’t say how they function—or how axial parenchyma works, or how the two systems interact. Textbooks want students to know that rays exist, but they don’t want students to know how they work. Does that make sense?