Evolutionary Significance of Ring Porosity in Woody Angiosperms

The problem of the evolutionary significance of the ring porous type of vessel arrangement has been investigated. Two methods were employed, one based on that of Jeffrey and the other on the correlation method of Bailey and his students. Both methods gave identical results, indicating that they are both valid in this type of problem. 2. Data from the anatomy of the seedling, first annual ring, root, and reproductive axis show that diffuse porosity is the more primitive condition for trees which are ring porous in the adult stem. 3. A high degree of correlation has been shown to exist between ring porosity and the presence of structural features whose advanced nature has been generally conceded. These are the simple perforation of the vessel segment end wall, the paratracheal distribution of wood parenchyma, and the presence of only simple pits on the wood fibers. The ray type does not show this correlation. From these data it is concluded that ring porosity is an advanced feature. 4. The evidence indicates that this specialization took place early in angiosperm history and affords no proof for the existence of parallel lines of evolution. Ring porosity has probably developed as a response to the climatic conditions characteristic of the North Temperate zone. These conditions are now peculiar to a wide but delimited region of the world, which also represents the modern range of the specialized type. Any morphological considerations of this structural specialization are valid only within the limits of that range.