Medullary Bundles and the Evolution of Cacti

The allometry of plant height H with respect to mean stem diameter D was determined based on 118 saguaro plants. The slope obtained for the reduced major axis regression analysis of the data was 2.36 ± 0.085, indicating that taller plants are disproportionately more slender than their shorter, presumably younger counterparts. The consequences of this positive, extremely anisometric relation on the elastic stability of stems were estimated by computing the critical buckling height H ent for each ofthe 118 stems on the basis of the mean density-specific stiffness (i.e., the quotient of Young's elastic modulus E and bulk tissue density p) determined for a single section from a mature saguaro stem. E1p was nearly equivalent to that of tissue samples of sclerenchyma isolated from other plant species. Since the slope of Hen. vs. D equals 2fl .., 0.67 when E1p .., a constant, the safety-factor for saguaro stems (i.e., Hen/H) appeared to be size-dependent such that it decreased with increasing plant height (i.e., Hen/H Q( D~'69). However, the mean safety-factor computed for the 118 saguaro specimens was 9.64, indicating that, on the average, plant height was well below Henr-Additionally, circumstantial evidence suggests that saguaro stems become more stiff as they increase in size (and age) and that the rate of stem growth decelerates over time. The former would obtain a near size-independent safety-factor against elastic buckling while the latter protracts the time required to reach the critical buckling height. Comparisons among the allometries of Hand H crit for saguaro, other cacti, nonwoody, and highly branched tree species indicated that saguaro size overlaps with the lower size-range of the largest known dicot and gymnosperm tree specimens likely as a consequence of the high EIp of mature saguaro stems.

Section: Discussionmentioning

confidence: 99%

mentioning

confidence: 99%

The allometry of saguaro height

Niklas

Buchman

1994

“…Studies that investigated anatomy and development of storage roots and succulent stems in groups outside Cactaceae are rare. The evolution of vascular strands such as cortical bundles (strands in the cortex), medullary bundles (strands in the pith), or strands in the axial system occurred in a variety of succulent groups with wide stems ( Olson, 2003 ;Mauseth, 1993Mauseth, , 2004Mauseth and Sajeva, 1992), but they were not observed in relatively thin seedlings of succulent Pelargonium (Geraniaceae; Jones and Price, 1996 ). Additionally, proliferation of parenchyma has been observed in succulent Fouquieriaceae ( Carlquist, 2001 , p. 276), Euphorbia ( Steinmann, 2001 , in the cortex), and Pelargonium ( Jones and Price, 1996 ).…”

Section: Generality Of Observations Witnessed Inmentioning

confidence: 99%

Developmental patterns in anatomy are shared among separate evolutionary origins of stem succulent and storage root‐bearing growth habits in Adenia (Passifloraceae)

Hearn

2009

The architecture of flowering plants is astonishingly diverse. To understand evolutionary patterns and processes that account for this diversity, I investigated developmental anatomy of storage roots and stems of 58 species in the genus Adenia (Passifloraceae) using an explicit phylogenetic context. Because expanded storage roots and stem succulence evolved multiple times in Adenia, patterns of transition between succulent and nonsucculent forms were analyzed using a comparative test that accommodates phylogenetic uncertainty. I tested the innervation hypothesis, wherein I expected the evolution of vascular strands to be correlated with evolutionary increases in water storage tissue if evolution of vascular strands facilitates transport through water and starch storage structures. Not only is evolution of vascular strands in stems statistically coupled with evolutionary increases in parenchyma storage tissue, most lineages that evolved expanded storage roots also evolved vascular strands in these roots in parallel to succulent stems. I proposed that vascular strands and closely associated storage parenchyma found in both roots and shoots of Adenia comprise a homologous unit. A switch-like evolutionary mechanism that alters the spatial expression of this unit between roots and shoots can account, in large part, for transitions between markedly different habits such as storage-rooted herbs and succulent-stemmed shrubs.

“…Medullary bundles have evolved multiple times in the history of vascular plants, being present in ferns (e.g., Cyatheaceae and Dennstaedtiaceae [ Pteridium ], Ogura, , ; Eames and MacDaniels, ; Lucansky, ), and more frequently in the flowering plants, where they have been recorded in approximately 60 families, including magnoliids (Isnard et al., ; Trueba et al., ) and eudicots (Wilson, ; Lambeth, ; Boke, ; Holwill, ; Metcalfe and Chalk, ; Davis, ; Esau, ; Pant and Bhatnagar, ; Raj and Nagar, , ; Kirchoff and Fahn, ; Mauseth, , ; Costea and DeMason, ; Schwallier et al., ; Kapadane et al., ). In some families, this character is found in just a few representatives (e.g., Nepenthes , Nepenthaceae; Schwallier et al., ) or is restricted to a specific lineage (subfamily Cactoideae, Cactaceae; Mauseth, , ; Terrazas and Arias, ), while in others it seems to be widespread within the family (e.g., Amaranthaceae, Nyctaginaceae, Piperaceae) (Metcalfe and Chalk, ). However, comparative analyses or investigations within a phylogenetic context are still lacking, except for Cactaceae (Mauseth, , ).…”

mentioning

confidence: 99%

Diversity, distribution, development, and evolution of medullary bundles in Nyctaginaceae

Neto

Pace

Douglas

et al. 2020

Premise Medullary bundles, i.e., vascular units in the pith, have evolved multiple times in vascular plants. However, no study has ever explored their anatomical diversity and evolution within a phylogenetic framework. Here, we investigated the development of the primary vascular system within Nyctaginaceae showing how medullary bundles diversified within the family. Methods Development of 62 species from 25 of the 31 genera of Nyctaginaceae in stem samples was thoroughly studied with light microscopy and micro‐computed tomography. Ancestral states were reconstructed using a maximum likelihood approach. Results Two subtypes of eusteles were found, the regular eustele, lacking medullary bundles, observed exclusively in representatives of Leucastereae, and the polycyclic eustele, containing medullary bundles, found in all the remaining taxa. Medullary bundles had the same origin and development, but the organization was variable and independent of phyllotaxy. Within the polycyclic eustele, medullary bundles developed first, followed by the formation of a continuous concentric procambium, which forms a ring of vascular bundles enclosing the initially formed medullary bundles. The regular eustele emerged as a synapomorphy of Leucastereae, while the medullary bundles were shown to be a symplesiomorphy for Nyctaginaceae. Conclusions Medullary bundles in Nyctaginaceae developed by a single shared pathway, that involved the departure of vascular traces from lateral organs toward the pith. These medullary bundles were encircled by a continuous concentric procambium that also constituted the polycyclic eustele, which was likely a symplesiomorphy for Nyctaginaceae with one single reversion to the regular eustele.