EARLY SENESCENCE OF CORTICAL CELLS IN THE ROOTS OF CEREALS. HOW GOOD IS THE EVIDENCE?

There are numerous reports that cortical cells senesce in young, otherwise healthy main roots of cereals, including corn. These are based on apparent absence of nuclei in root segments or transverse sections after acridine-orange staining. Senescence is said to progress from the outer to the inner cortex basipetally from the root tip, except cells around branch bases where nuclei always stain. We studied axile roots of soil-grown cereals using various methods to detect nuclei primarily in longitudinal sections. No senescence marked by nuclear loss was found in healthy-looking intact cortices. Cortical cells of mature corn roots remained alive except where aerenchyma developed. No cortical death had occurred in barley, wheat, or oat seminal roots in 15-,17-, and 20-day-old plants, respectively, but cortical cells in older regions of seminal and nodal roots did collapse and slough off, but with no evidence for earlier loss of nuclei. Failure to detect acridine-orange-stained nuclei may not indicate that cells are senescent, and can be an artifact caused by sectioning method and wall impermeability. The effectiveness of other methods for evaluation of root cell vitality is discussed.     

野生华南虎是怎样走向濒危的--兼谈华南虎保护拯救之希望

近20年来,关于华南虎的数量及保护、拯救工作的报道频频见于各种学术期刊、杂志和报纸、广播、电视新闻中,近几年来更常见于Internet网。有关华南虎野生数量的变化众说纷纭,多是“估计”值,本文就各估计值做一归纳统计,以阐述其走向濒危的过程,分析原因,并建议在目前国家财力不是十分雄厚的情况下,华南虎的保护拯救工程应积极寻求国际资金的支持。     

HOW TO COMPENSATE FOR COSTLY SEXUALLY SELECTED TAILS: THE ORIGIN OF SEXUALLY DIMORPHIC WINGS IN LONG-TAILED BIRDS

Recent work on birds suggests that certain morphological differences between the sexes may have evolved as an indirect consequence of sexual selection because they offset the cost of bearing extravagant ornaments used for fighting or mate attraction. For example, long-tailed male sunbirds and widowbirds also have longer wings than females, perhaps to compensate for the aerodynamic costs of tail elaboration. We used comparative data from 57 species to investigate whether this link between sexual dimorphism in wing and tail length is widespread among long-tailed birds. We found that within long-tailed families, variation in the extent of tail dimorphism was associated with corresponding variation in wing dimorphism. One nonfunctional explanation of this result is simply that the growth of wings and tails is controlled by a common developmental mechanism, such that long-tailed individuals inevitably grow long wings as well. However, this hypothesis cannot account for a second pattern in our data set: as predicted by aerodynamic theory, we found that, comparing across long-tailed families, sexual dimorphism in wing length varied with tail shape as well as with sex differences in tail length. Thus, wing dimorphism was generally greater in species with aerodynamically costly graduated tails than in birds with cheaper, streamer-shaped tails. This result was not caused by confounding phylogenetic effects, because it persisted when phylogeny was controlled for, using an independent comparisons method. Our findings therefore confirm that certain aspects of sexual dimorphism may sometimes have evolved through selection for traits that reduce the costs of elaborate sexually selected characters. We suggest that future work aimed at understanding sexual selection by investigating patterns of sexual dimorphism should attempt to differentiate between the direct and indirect consequences of sexual selection.