The structure and development of interxylary phloem (IP) and external phloem in Aquilaria sinensis were investigated using light and scanning electron microscopy. Complete IP strands were isolated, measuring 14 ± 4 mm in length and 417 ± 124 μm in width. The outer margin of IP was composed of two to three layers of fusiform parenchyma cells. The development of IP can be divided into five stages: 1) Locally IP starts its differentiation within a small segment of a broad cambial zone, at the cost of xylem differentiation. 2) Inward growth of IP advances, and fibres and sieve tubes differentiate. 3) IP is constricted by the encroachment of immature xylem cells between cambium and immature IP. 4) IP is isolated from the cambium and surrounded by immature, non-lignified xylem tissue. 5) IP is surrounded by lignified xylem tissue, and the fibres within IP become lignified.
In all the phloem islands in a ten-year-old stem, sieve elements showed positive staining of callose with aniline blue. However, no staining of callose was observed in the external secondary phloem of agarwood trees collected from two different sites. No sieve tubes or sieve pores were detected by SEM observation of numerous serial cross and radial sections of the external phloem. We therefore conclude that sieve tubes are absent from the external phloem or extremely rare and that the transport of photosynthetic products in the stem of A. sinensis takes place in the interxylary phloem.
Agarwoods such as Aquilaria spp. and Gyrinops spp. (Thymelaeaceae) produce interxylary phloem in their secondary xylem and intraxylary phloem at the periphery of the pith, facing the primary xylem. We studied young shoots of Aquilaria sinensis and characterized the development of its intraxylary phloem. It was initiated by the division of parenchyma cells localized in the outer parts of the ground meristem immediately following the maturation of first-formed primary xylem. Its nascent sieve plates bore donut-like structures, the individual pores of which were so small (less than 0.1 μm) that they were hardly visible under FE-SEM. Intraxylary phloem developed into mature tissue by means of the division and proliferation of parenchyma cells. During the shoots’ active growth period, the sieve pore sizes were 0.1–0.5 μm, with tubular elements passing through them. In the maturation stage, large clusters of sieve tubes continued to be differentiated in the intraxylary phloem. In the partial senescence stage observed in a three-centimeter-diameter branch, intraxylary phloem cells in the adaxial part became crushed, and sieve plates had pores over 1–2 μm in diameter without any callose deposition. Before and after the differentiation of interxylary phloem in the first and second internodes, callose staining detected more than twice as many sieve tubes in intraxylary phloem than in external phloem. However, after differentiation of interxylary phloem in the eleventh internode, more sieve tubes were found in interxylary phloem than in intraxylary and external phloem. This suggests that prior to the initiation of interxylary phloem intraxylary phloem acts as the principal phloem. After its differentiation, however, interxylary phloem takes over the role of principal phloem. Interxylary phloem thus acts as the predominant phloem in the translocation of photosynthates in Aquilaria sinensis.
We have used phylogeographic methods to investigate the genetic structure and population history of the endangered Himalayan snowcock (Tetraogallus himalayensis) in northwestern China. The mitochondrial cytochrome b gene was sequenced of 102 individuals sampled throughout the distribution range. In total, we found 26 different haplotypes defined by 28 polymorphic sites. Phylogenetic analyses indicated that the samples were divided into two major haplogroups corresponding to one western and one eastern clade. The divergence time between these major clades was estimated to be approximately one million years. An analysis of molecular variance showed that 40% of the total genetic variability was found within local populations, 12% among populations within regional groups and 48% among groups. An analysis of the demographic history of the populations suggested that major expansions have occurred in the Himalayan snowcock populations and these correlate mainly with the first and the second largest glaciations during the Pleistocene. In addition, the data indicate that there was a population expansion of the Tianshan population during the uplift of the Qinghai-Tibet Plateau, approximately 2 million years ago.
New observations of radial sieve tubes in the secondary xylem of two genera and four species of agarwood — Aquilaria sinensis, A. crasna, A. malaccensis and Gyrinops versteeghii (Thymelaeaceae) — are presented in this study. The earliest radial sieve tubes in Gyrinops are formed in the secondary xylem adjacent to the pith. The radial sieve tubes originate from the vascular cambium and develop in both uniseriate and multiseriate ray tissue. In addition to sieve plates in lateral and end walls, scattered or clustered minute sieve pores are localized in the lateral wall of radial sieve tubes. There is a direct connection between radial sieve tubes in ray tissue and axial sieve tubes in interxylary phloem strands (IP), such as (i) connection by bending of radial sieve tube strands, (ii) connection of two IP strands by an oblique bridge, and (iii) connection of two IP strands at a right angle. The average number of radial sieve tubes and interxylary phloem was found to be 1.7 per mm3 and 9.1 per mm2 in the secondary xylem. Considering the higher frequency of radial sieve tubes with the increasing thickness of the secondary xylem, the direct connections between radial and axial sieve tubes could play a significant role in assisting the translocation of metabolites in Aquilaria and Gyrinops.
Brain size varies dramatically between vertebrate species. Two prominent adaptive hypotheses – the Cognitive Buffer Hypothesis (CBH) and the Expensive Brain Hypothesis (EBH) – have been proposed to explain brain size evolution. The CBH assumes that brain size should increase with seasonality, as the cognitive benefits of a larger brain should help overcoming periods of food scarcity via, for example, increased behavioral flexibility. Alternatively, the EBH states that brain size should decrease with seasonality because a smaller brain confers energetic benefits in periods of food scarcity. Here, to test the two adaptive hypotheses by studying the effects of variation in temperature and growth season on variations in overall brain size and the size of specific brain regions (viz. olfactory nerves, olfactory bulbs, telencephalon, optic tectum and cerebellum) among Hylarana guentheri populations. Inconsistent with the predictions of both the EBH and the CBH, variation in temperature and growth season did not exhibit correlations with overall brain size and the size of brain regions across populations. Hence, our data do not provide support for either the EBH or the CBH to explain brain size variation in H. guentheri. Furthermore, brain size variation did not differ between males and females in this species. Our findings suggest that both the variation in temperature and growth season did not shape the variation in brain size in H. guentheri.