Robert F. Parsons

The majority of taxa with peltate leaves are perennial herbs native to swampy or aquatic habitats or to mesic shaded understorey habitats. These large peltate leaves are formed by a meristematic bridge at the lamina–petiole junction. However, there are also several strong-light exposed, small-leaved, xero- and scleromorphic Myrtaceae with leaf peltation which is formed without a meristem fusion/bridge. Here, abaxial laminar tissue at the insertion point of the petiole forms a basal extension, so that a weak peltation occurs. This shifts the petiole onto the adaxial laminar surface. The formation of micropeltation in Myrtaceae leads to erect leaves that are strongly appressed to the shoot axis and the entire foliate, vertical shoots appear as “green columns”, a result that is also the case in taxa with reflexed minute leaves. It seems that micropeltation achieves the same goal as leaf reflexion in small-leaved taxa—reduction of heat-load and transpiration during the hottest phases of the day by a lower light interception at midday compared to the morning and evening. Thus, physiologically micropeltation and reflexion of minute leaves seem to be the result of convergent evolution.

Key message: We examined leaves of a suite of microphyllous woody plants and describe a little-known form of leaf peltation for the first time and also investigate strongly reflexed leaves in two distantly related lineages. Plants cope with a range of environmental conditions, especially related to water relations, and have developed an array of physiological and structural solutions to maintain a functional water balance. There has been considerable recent work on physiological solutions to water deficit but little attention paid to leaf characteristics. In many species there is a change in leaf form from seedlings to adults. We examine such changes in several small-leaved species from the distantly related Asteraceae and Myrtaceae, some of which develop micropeltate or reflexed leaves as adults. All are native to dry or seasonally dry sites. Three major morphological groups were recognised as follows: (1) leaves erect, nonpeltate and scale-like (Ozothamnus hookeri), (2) leaves erect and peltate (Phaenocoma prolifera,Regelia inops), (3) Leaves reflexed (Olearia lepidophylla,Ozothamnus scutellifolius,Ozothamnus reflexifolius,Melaleuca diosmifolia). The microphyllous peltation inP. proliferaandR. inopsin the absence of a meristematic fusion/bridge differs from typically peltate leaves. These small-leaved taxa occur in open, high light environments which are very different from the mesic, shaded understorey habitats of typical peltate-leaved plants. Many small-leaved species have leaves closely appressed to the stem and often with recurved margins. The erect leaves are functionally similar to reflexed leaves. Environmental filtering leads to superficially similar plant forms that may have somewhat different ontological origins. Such morphological forms are examples of convergent evolution in distantly related species but within each family are likely phylogenetically related.

Key message: We examined the leaf structure of two basal Cupressus species with a distinct leaf dimorphism. Some foliar features are regarded as ancestral and it is suggested that drought adaptation was one of the important ecological drivers in the evolution of the Cupressus genus. Leaf morphology and anatomy of two Cupressus species, C. nootkatensis and C. vietnamensis, were investigated with classical paraffin technique and scanning electron microscopy (SEM). Like all Cupressus species these two are characterised by a dramatic change in the foliage. Juveniles have needle leaves first before they change abruptly to the mature scale leaf type. In C. vietnamensis, needle-leaved shoots occur next to scale-leaved ones even on mature trees, which is unique among today´s Cupressus species. Adults of C. nootkatensis develop only scale leaves throughout. In both taxa, the scale leaves show a distinct dimorphism between lateral and facial leaves, which are arranged in a flat spray; the foliate shoots are two-dimensionally flattened. These scale leaves show several xeromorphic features; e.g. strongly reduced leaf size, stomata with high, collar-like Florin rings, the presence of a distinct hypodermis as a continuous layer and well-developed transfusion tissue. The needle leaf type is found in Cunninghamia which is the basal member of the Cupressaceae and so is regarded as the ancestral condition and scale leaves as a derived one. Scale leaves are found in all the members of the cupressoid clade even within the basal taxa from mesic habitats. However scale leaves are a preadaptation to survival under xeric conditions and they are likely an evolutionary driver of the radiation of Cupressus into arid environments, as has also been the case in genera such a Callitris.

Recently, a list has been published of angiosperms capable of germinating in less than 24 h (‘very fast germination’). Here, we add three families and 23 species to that list. The main extra families complement ecological groups already recognized – the Cactaceae into the aridity-adapted group and the Tamaricaceae into the floodplain-adapted group. These amended findings on very fast germination (VFG) are integrated into the recent work on the functional ecology of embryo size. They confirm the important connection between germination speed and embryo to seed ratio. Plotting the plant orders containing VFG species on a phylogenetic tree shows that VFG has evolved multiple times throughout angiosperm history, including at least three times within the Caryophyllales. The fact that species with VFG are mainly restricted to advanced clades shows that VFG is a derived trait that evolved as an adaptation to either arid, saline or floodplain habitats.

A group of flowering plant species is known to germinate in less than 24 h from imbibition, but this phenomenon is often overlooked in the current literature. Here, I review this topic by searching the literature published since 1967 and listing the 28 most detailed cases found. Of these, 20 are species of Amaranthaceae (all formerly treated as Chenopodiaceae); 15 of these are from the subfamily Salsoloideae, which is characterized by the possession of spiral embryos. The non-chenopods listed are small numbers of species from the families Acanthaceae, Cruciferae, Gramineae (one species) and Salicaceae (PopulusandSalix). Seeds of the Salsoloideae contain fully differentiated embryos. On imbibition, the embryo cells elongate and the spiral embryo uncoils and ruptures the thin seed coat. This can occur in as little as 10 min. Nearly all of the families showing very fast germination have small to very small seeds and little or no endosperm. Most species have soft, thin seed coats that imbibe water readily. All are from high-stress habitats, either arid or saline or from active floodplains, where they can rapidly exploit temporarily favourable conditions for germination. They exhibit one of two contrasting germination behaviours, either having seeds which all germinate within a very short time of wetting or having seed persistence whereby small amounts of rain cause germination of small fractions of seed from a long-living soil seed bank. Serious confusion in the literature in the use of the term ‘opportunistic’ is pointed out and clarified.