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Darwin cast his net widely to accumulate evidence in support of his belief in organic evolution by natural selection. That net inevitably caught some succulents, including those he saw face to face in South America and the Galapagos Islands. Here for the first time his references are brought together and evaluated. They provide an object lesson to growers today in how to increase interest and advance knowledge by looking closer and asking the right questions.
Succulence is an adaptive strategy that allows plants to remain active during seasonal water shortage. The term was first used formally by Johann (Jean) Bauhin in 1619 to refer to plants with thick, juicy leaves. Its subsequent use and selected definitions are critically discussed, including concepts such as utilizable water, caudiciforms and pachycauls, and root succulence. A unified definition of succulence considers aspects of morphology and anatomy, ecology, and physiology. Stem succulence and the “cactus life form” are used to illustrate the parallel evolution of functional adaptations in morphology, and to contrast the obvious external similarities with the widely variable internal architecture, including the participation of different stem tissues in water storage.
Since at least 1930, evolutionary explanations have focused on Darwin's mechanism for driving evolution, namely natural selection (and one of its side effects, adaptation), mostly neglecting other evolutionary forces. Yet, genetic drift probably plays a huge role in plant evolution, as does drift's supporting cast of gene-gene interactions and inbreeding. I argue that cactus evolution should be largely driven by drift, rather than selection, because of high rates of polyploidy, inbreeding, and endemism, which collectively result in highly reduced effective population sizes for virtually all of the Cactaceae.
Evolution of habit in Cactaceae is of much recent interest, especially with regard to leaf form and below ground storage. Recent study of Opuntioideae provides a phylogenetic framework for considering morphological evolution in that subfamily. Four main lineages exist within the Opuntioideae. The deepest lineage consists of early deciduous, globular-stemmed, diminutive, often geophytic plants (Maihueniopsis). The second deepest lineage consists of early deciduous, globular- or cylindricalstemmed geophytes (Pterocactus). Persistent leaves are a derived, iterative character state in Opuntioideae. Diversity in Opuntioid morphological features (below ground storage organs, stem shape, and leaf persistence) in the phylogenetic context shows a trend towards increased size and surface area. Parsimonious reconstruction of ancestral character states suggests that early Opuntioideae may have been diminutive, early-deciduous, possibly geophytic plants. The polyphyletic genera Maihueniopsis and Puna (= Maihueniopsis s.l.) possess many characters plesiomorphic for Opuntioideae.
The present study focuses on flower visitors and pollinators of some globular cacti and opuntias of the southwestern USA. Flowers of these cacti are predominantly melittophilous, and medium-sized to large bees are their most important and most efficient pollinators. A brief summary of the most common bee genera visiting cactus flowers in southwestern USA is given. Other common visitors of the cactus flowers are nitidulid beetles that are very poor pollinators and can be considered pollen thieves. Two other unexpected cactus flower visitors were identified during this study: true flies of the genus Triploechus from Pediocactus bradyi flowers and hawk moth (Hyles lineata) from a flower of Echinomastus johnsonii.
Nectaries of flowers from all four subfamilies of the Aizoaceae have been studied in detail under the scanning electron microscope (SEM), permitting the description of a new nectary system. The Ruschioideae can be distinguished by their bulging nectaries from all others with a plain nectary type, into which the coilomorphic nectaries of the Mesembryanthemoideae fall as well. Notable is that the plain nectaries of the Aizooideae sit apically in the tubes of the perigynous flowers, probably representing a synapomorphy of the subfamily. A subdivision of the Ruschioideae cannot be undertaken based on nectary features only. Instead, it is suggested that the precise shape and size of the bulging nectary is not influenced alone by space available in the flower but also by a competition for space in the same area by characters of the capsules, especially the abutment of the valves. Pollination and dispersal are strongly exposed to selection, and both are kept in balance in different ways in each species or genus, asking for extensive studies in as many species as possible in order to determine evolutionary traits.
Cactus spines from 21 species out of two subfamilies, six tribes and 14 genera were examined by electron microscopy to determine their structural properties. In addition, the spines were tested for their bending stiffness and the results compared with the structural properties. All the spines observed were built of longitudinally oriented fibrous structures. However, they clearly showed two different construction principles. Some spines were built of sheets, placed in layers with different fibre orientation and showing a folded appearance in cross section. Others were built from pillar-like structures, where the main building blocks consisted of longitudinally oriented strands of fibres. Where those spines with a folded structure were mainly (8 out of 11) curved, those with a pillar structure were straight (9 out of 10). The range of bending stiffness, in the spines examined, ranged from 0.007 GPa up to 12.7 GPa. These values were compared to those for common materials, from rubber to hard wood. These findings could be used for taxonomy or could serve as templates for man-made composite structures.
A watercolour of Stapelia hirsuta by the little-known Dutch artist Nicolaas Juweel (c. 1639–1704) made a brief public appearance in 1998 at The European Fine Art Fair, Maastricht. Its provenance is uncertain, but it is suggested that the drawing was commissioned by Agnes Block, art collector and owner of a large plant collection, on her estate Vijverhof near Utrecht between 1675 and 1697. It is currently in a private collection in Germany.
Perrier de la Bathie (1926) described Aloe deltoideodonta var. intermedia Perrier for a plant collected W. of Betroka, Madagascar. Later Reynolds (1957) mistakenly concluded that a plant collected N. of Zazafotsy, in the highlands of Madagascar, also represented Perrier's variety, but believed it deserved specific status and published the new combination A. intermedia (Perrier) Reynolds. Later it was shown that this name is invalid because it is predated by A. intermedia Haworth, so the replacement name A. subacutissima Rowley was published. Here it is shown that the plant described as A. intermedia (Perrier) Reynolds is not the same as Perrier's original variety, and it is redescribed as a new species, A. newtonii J.-B. Castillon.
The original description of Sansevieria arborescens was incomplete, and the name was misapplied by Brown in his 1915 monograph. The suggestion that S. arborescens and S. bagamoyensis might be conspecific has yet to be supported by convincing evidence. Plants matching the protologue of S. arborescens have flowered in cultivation, and an amplified description is provided. Some plants in Kenya that were erroneously called S. arborescens are described as a new species, S. dumetescens, distinguished by their longer and recurved leaves, and larger inflorescence.
Evidence is presented to show that Aloe teissieri Lavranos was effectively published before Aloe andohahelensis J.-B. Castillon, and since these two taxa are now considered to be conspecific, the former name has priority over the latter.
A new species from Thailand, Ceropegia thailandica Meve (Apocynaceae-Asclepiadoideae-Ceropegieae) is described and illustrated. The new species is related to the Indian C. spiralis, and belongs to a group of tuberous, non-twining, grass-like species that are widespread on the Indian subcontinent. The single, exeedingly long-stalked flowers with ovoid basal inflations and very long, tightly spiralled corolla lobes, leaving only small and characteristically auriculate openings for insect visitors, make the new species an unrivalled appearance in Ceropegia.
Two species of Aloe L., Aloe vera (L.) Burm.f. and A. arborescens Mill. have been recorded as naturalised in Portugal: A. vera as an occasional escape along the Algarve in the south, and A. arborescens from central coastal regions around Lisbon. Here we record the spread of A. arborescens further north along the Portuguese coast, especially around the coastal town of Nazaré. This species has become firmly established as part of the introduced flora of the country. Its potentially serious impact as an invasive species is emphasised.