A SYSTEMATIC ANATOMICAL STUDY OF THE GENUS PERIDERIDIA (UMBELLIFERAE‐APIOIDEAE)

Certain anatomical features, especially of roots, leaves, and fruits, provide useful criteria for delimiting a genus and defining its component species. Most species of Perideridia have 1‐5 multistelic tuberous roots; however, P. Howellii and P. Kelloggii possess fascicles of up to 20 thickened fibrous roots, each of which is monostelic and composed of four or five primary xylem strands arranged alternately with isolated strands of secondary vascular tissues. The curious multistelic condition of the tuber apparently has not been reported for any other group of dicotyledons. An accessory vascular bundle located internally to the median collateral bundle of the leaf rachis is found in only three species of Perideridia . Depending on the species, the leaf mesophyll may be either dorsiventral or isolateral. In about two‐thirds of the species, the oil ducts of the fruit are solitary in the intercostal intervals and paired on the commissure; in the remaining one‐third they are more numerous in both situations, rising to 3‐5 in the intervals and 12‐13 on the commissure in P. Pringlei . Variation in number of oil ducts in the intervals assumes taxonomic importance because it has been used as a generic criterion in this group, but the present study shows no correlation between the number of oil ducts and other characters.