Abstract

The anatomical structure of the leaves and stems of the 13 species of Isostigma (Asteraceae: Heliantheae) has been examined by using light microscopy. All species of Isostigma have Kranz anatomy in their leaves, containing one or more Kranz units (=KU, the unit constituted by the vascular bundle/s, the parenchyma sheath and the surrounding mesophyll). It is demonstrated that there are the following two different types of Kranz anatomy in leaves of Isostigma: (1) Eryngiophyllum type, with one KU per leaf and with sclerenchyma tissue (I. brasiliense, I. cordobense, I. crithmifolium, I. dissitifolium, I. peucedanifolium, I. riedelii, I. simplicifolium and I. speciosum); and (2) Isostigma type, with more than one KU per leaf, without sclerenchyma tissue (I. acaule, I. herzogii, I. hoffmannii, I. molfinianum and I. scorzoneraefolium). The stems of all 13 species of Isostigma show also Kranz anatomy, without variation among species. Until the present, the Eryngiophyllum and the Atriplicoid types were the only reported for Asteraceae. The Isostigma type is a new type for the family, characteristic of plants growing in humid places. The following evolutionary sequence of Kranz anatomy is hypothesised: Atriplicoid–Isostigma–Eryngiophyllum where numerous KUs become continuous to reach a unique, compound KU.