Unique shoot architecture in the leafy liverwort Herzogianthus vaginatus (Herzogianthaceae): insights into novel growth patterns in Jungermanniidae

Abstract

All studied leafy liverworts have shoots with leaf to underleaf ratios of either 2:1 or 1:1. These ratios are the product of growth by either helical or pendular segmentation of the tetrahedral apical cell. Here we report that Herzogianthus vaginatus has a leaf to underleaf ratio approaching 3:1 on primary shoots, and on secondary shoots the ratio is closer to 4:1. These ratios are incompatible with the simple helical or pendular growth patterns found in other leafy liverworts. Further, the sequence of leaves and underleaves on Herzogianthus shoots is not wholly regular, which is previously unrecorded. We assess potential growth models that may explain the leaf to underleaf ratios and architecture of Herzogianthus shoots, including (1) branch modification, (2) merophyte or leaf-initial duplication and (3) changes in segmentation sequences. We compared leaf sequences produced by pendular and helical segmentation and a range of non-standard models, with sequences observed on shoots of Herzogianthus. Structure allows us to discriminate between the many alternative growth models, because each make testable predictions about leaf and underleaf sequences. We propose that Herzogianthus possesses growth processes unique among leafy liverworts, in which the direction of helication periodically and, sometimes irregularly, reverses. This, in combination with the secondary shoots having different growth patterns from primary shoots, emphasizes that Herzogianthus warrants detailed histological study to test our proposition.