James D. Mauseth

Theoretical aspects of surface-to-volume ratios and water-storage capacities of succulent shoots

Surface-to-volume (S/V) ratios of drought-adapted plants affect transpiration, photosynthesis, and water-storage capacity. The S/V ratio of cladodes and flat leaves is S/V = 2/T, where T is thickness: even slight thickening greatly reduces S/V. During rain/drought cycles succulent stems swell and shrink without tearing by having flexible ribs, but ribs increase S/V above that of a smooth cylindrical stem with equal volume: the increased surface area is S(ribbed)/S(cylindrical) = N[x + (π/N)]/π(1 + x), where N is number of ribs and x is rib height relative to the radius of the inner stem. Numerous low ribs provide moderate expandability (storage volume) with little increase in S/V and are adaptive where droughts are short. Tall ribs provide greater expandability but greatly increase S/V and probably are adaptive only in mesic habitats. Having ∼8-15 ribs, each about as tall as the inner stem radius, provides large storage capacity and intermediate increase in S/V. By increasing absolute size, S/V is reduced so greatly that even large ribs can have an S/V smaller than that of a narrow cylindrical or spherical stem with less volume.

Relictual vegetative anatomical characters in Cactaceae: the genusPereskia

The genusPereskia, which contains numerous morphological features considered relictual in the Cactaceae, has numerous anatomical features that we consider to be relictual also. These were studied to establish a basis for determining the ways that morphogenic mechanisms and anatomical characters diversified as the family evolved. ThesePereskia features may be relictual in the family: epidermis predominantly unistratose and lacking crystals; hypodermis absent or of about three layers of weakly collenchymatous cells with druses; cortex thin and predominantly parenchyma with druses and mucilage cells but lacking cortical bundles; secondary phloem without early differentiation of sclerenchyma but with secondary sclereids developing later, either idioblastically or in clusters; ergastic substances lacking from old secondary phloem; wood with a matrix of libriform fibers (mostly septate and nucleate), scanty paratracheal parenchyma, vessels solitary or in small clusters, perforations simple, pitting circular, oval or very broad; wide-band tracheids absent; ray cells slightly thick-walled, lignified, upright, isodiametric or procumbent; all primary rays narrow; pith without medullary bundles; leaves lacking hypodermis, with only weak development of palisade mesophyll; veins of four orders, strongly distinct in size, none with fibers; vessels in leaves narrower than those in stems.

Structural and systematic study of an unusual tracheid type in cacti

Wide-band tracheids are a specialized tracheid type in which an annular or helical secondary wall projects deeply into the cell lumen. They are short, wide and spindle-shaped, and their bandlike secondary walls cover little of the primary wall, leaving most of it available for water diffusion. Wide-band tracheids appear to store and conduct water while preventing the spread of embolisms. They may be the most abundant tracheary element in the xylem, but they are always accompanied by at least a few vessels. Typically, fibers are absent wherever wide-band tracheids are present. Wide-band tracheids occur in the primary and secondary xylem of succulent stems, leaves and roots in genera of all three subfamilies of Cactaceae but were not found in the relictual genusPereskia, which lacks succulent tissues. In the large subfamily Cactoideae, wide-band tracheids occur only in derived members, and wide-band tracheids of North American Cactoideae are narrower and are aligned in a more orderly radial pattern than those of South American Cactoideae. Wide-band tracheids probably arose at least three times in Cactaceae.

Anatomical Adaptations to Xeric Conditions in Maihuenia (Cactaceae), a Relictual, Leaf-Bearing Cactus

Maihuenia and Pereskia, constitute Pereskioideae, the subfamily of Cactaceae with the greatest number of relictual features, but the two genera differ strongly in habit and ecological adaptations. Plants of Maihuenia occur in extremely xeric regions of Patagonia and are small cushion plants with reduced, terete leaves and soft, slightly succulent trunks. Plants of Pereskia occur only in mesic or slightly arid regions and are leafy trees with hard, woody trunks and thin, broad leaves. Maihuenias have many more anatomical adaptations to arid conditions than do pereskias: maihuenias lack sclerenchyma in their phloem and cortex (M. poeppigii also lacks xylem sclerenchyma and can contract during drought); their wood consists of vessels, axial parenchyma, and wide-band tracheids and can store water as well as minimize embolism damage; one species channelizes water flow by producing intraxylary bark; and at least some stem-based photosynthesis occurs because maihuenias have small patches of persistent stem epidermis that bears stomata and overlies a small amount of aerenchymatous chlorenchyma. Pereskias lack all these features. Although closely related, maihuenias have fewer relictual features than do pereskias, and plants of Pereskia probably are more similar to the ancestral cacti.

Wide-band tracheids are present in almost all species of Cactaceae.

Wide-band tracheids (WBTs) have been found in seedlings of most species of cacti that have fibrous wood in their adult bodies. Consequently, this cell type is now known to be present in almost all cacti. Earlier studies of adult plants revealed WBTs to be present only in cacti with globose or short, broad bodies, whereas all species with large columnar or long slender bodies had fibrous wood without WBTs. However, even these species produce WBTs during the first several months after germination. In species with fibrous wood in their adult bodies (species with large or slender bodies), seedlings undergo a phase transition in wood morphogenesis after a few months and stop producing the juvenile (WBT) wood and begin producing adult (fibrous) wood. If adult plants have an intermediate size, the phase transition is delayed and the plant produces WBT wood for several years. Species with globose bodies repress the phase transition completely and never switch to producing adult (fibrous) wood. Because WBTs are so widespread, they probably originated only once in Cactaceae, not multiple times as suggested earlier, or there may have been just a single origin in the Cactaceae/Portulacaceae clade.

Comparative structure-function studies within a strongly dimorphic plant, Melocactus intortus (Cactaceae)

Summary. Melocactus intortus is a species of strongly dimorphic plants whose different parts show strong correlations between structure and function. Each individual has one unbranched monopodial shoot; during the first several years of life, this grows as a photosynthetic juvenile with a transparent epidermis, numerous stomata and a system of cortical bundles that permeate the chlorophyllous outer cortex. Cortical bundles undergo secondary growth. The xylem of the stele contains wide vessel elements, vascular tracheids and fibres, resulting in a wood that is probably relatively strong and efficient at water conduction. After several years, the plant undergoes a transition to being adult and all further shoot elongation growth results in a region of body with different anatomy and which is capable of flowering, the cephalium. The epidermis is thin, lacks stomata and is soon converted to a cork cambium. The cortex is nonphotosynthetic and cortical bundles do not have secondary growth. The wood of the adult portion contains only narrow vessels and parenchyma. The juvenile and adult portions of each shoot have distinct functions as well as distinct requirements for gas exchange; water and nutrient transport; and mechanical strength. When interpreting the selective value of the various anatomical features, it is critically important to consider the specific role and requirements of the particular portion of the plant in which they occur.

PRESENCE OF PARATRACHEAL WATER STORAGE TISSUE DOES NOT ALTER VESSEL CHARACTERS IN CACTUS WOOD

This research tested hypotheses that the presence of water storage tissues immediately adjacent to vessels would protect vessels from cavitation and would result in evolution of broader vessels that occur in fewer, smaller clusters relative to vessels surrounded by a matrix of fibers. We examined 21 species that have dimorphic wood, that is, at one stage in their life they produce a wood with a fibrous matrix surrounding the vessels and at another stage they produce wood with abundant paratracheal parenchyma or wide-band tracheids. In only one species were vessels in the water storage matrix broader than those in the fibrous matrix of the same plant. In most specimens, fibrous wood had smaller clusters of vessels than water storage wood, and a greater percentage of vessels in fibrous wood were solitary. Presence of abundant paratracheal water storage tissue was not correlated with a reduced number or size of rays. Axial masses in fibrous wood were not consistently narrower than those of water storage wood, consequently their vessels were not consistently closer to water stored in rays. Wood strength may be more important than conduction safety in determining vessel cluster size and widths of rays and axial masses.

THE STRUCTURE OF PHOTOSYNTHETIC SUCCULENT STEMS IN PLANTS OTHER THAN CACTI

In an investigation of succulence as an adaptation to water‐stressed desert environments, we studied the anatomy of photosynthetic succulent stems in 28 species from Apocynaceae, Asclepiadaceae, Asteraceae, Crassulaceae, Didieriaceae, Euphorbiaceae, Geraniaceae, and Vitaceae. Only species with enlarged pith or cortex were examined, not species with enlarged parenchymatous wood. The numbers of species with particular characters are as follows: pith is the most voluminous tissue—5 species; cortex is the most voluminous tissue—22; cortical bundles present—0; medullary bundles present—4; palisade cortex present—16; epidermis with thick or lignified walls—0; epidermis outer wall somewhat thickened—6; multiple epidermis present—0; cuticle >2 μm thick—4; cuticle <1 μm thick—3; hypodermis cells with very thick walls—0. These species lack many features typically considered xeromorphic and associated with desert‐adapted plants; they also lack many features present in most cacti. Absence of cortical bundles in all these noncactus succulents may be especially important: none has a particularly thick cortex (most are <5 mm thick), all are relatively small plants, and absence of cortex vascularization may prevent them from evolving to be truly voluminous. Also, none has truly large shoot apical meristems such as occur in cacti with broad shoots.

Comparative Anatomy of Tribes Cereeae and Browningieae (Cactaceae)

Summary: The purpose of this study was to assess the phylogenetic relationships between tribes Cereeae and Browningieae in the cactus subfamily Cactoideae. During field trips to Venezuela, Ecuador and Chile, tissue samples were collected from representative specimens in natural populations. Monvillea* diffusa, M. maritima, and M. smithiana were virtually identical anatomically and, although similar to Cereus hexagonus, they were distinct in having large druses in their outer hypodermis and extremely small wall crystals in their cortex. Pilosocereus mortensenii was indistinguishable from the samples of Subpilocereus* (S. ottonis, S. repandus, S. russelianus) but was very different from other pilosocerei (P. lanuginosus, P. moritzianus, P. tillianus). Samples of Subpilocereus, including P. mortensenii, were similar to those of C. hexagonus in having no hypodermal crystals, no druses in pith or cortex, few or no mucilage cells in pith or cortex. Anatomical characters supported a hypothetical close relationship between Armatocereus (A. brevispinus, A. cartwrightianus, A. godingianus) and Jasminocereus (J. thouarsii), but Jasminocereus had few distinctive features that could be used to link it strongly to any genus in this study. Both tribes were variable, but members of each resembled each other more than they resembled any member of the other tribe. Leptocereus quadricostatus (tribe Echinocereeae) did not obviously share more characters with one tribe versus the other. Pachycereeae are more similar to Cereeae than to Browningieae.

Systematic anatomy of the primitive cereoid cactus Leptocereus quadricostatus

Summary. Systematic anatomy of the primitive cereoid cactus Leptocereus quadricostatus. The genus Leptocereus is widely regarded as one of the most primitive of Cactaceae subfamily Cactoideae. It resembles the pereskias in having dense fibrous wood in its trunk and lower branches; this wood lacks vascular tracheids; it has septate protophloem fibres; and the ray cells are all thick walled and lignified. However, it also shows the beginnings of many features which are associated with the more advanced cacti: it has an enlarged, palisade cortex, a system of cortical vascular bundles, and medullary bundles. In addition, the wood of the uppermost portions of many branches is very parenchymatous, unlike the fibrous wood of the trunk. In some portions of the shoot, especially in the cephalia, the wood consists of just parenchyma and vessels, a trait which had been considered quite advanced. It seems reasonable that a strongly dimorphic plant such as Leptocereus would be able to give rise to the numerous diverse lines of evolution which exist in the Cactoideae.