Lancashire J R, Ennos A R
School of Biological Sciences, University of Manchester, 3.614 Stopford Building, Oxford Road, Manchester M13 9PT, UK.
J Exp Bot. 2002 Jun;53(373):1485-93.
Previous studies of the hydrodynamics of plant stems have shown that resistance to flow through bordered pits on the side walls of tracheids makes up a significant proportion of their total resistance, and that this proportion increases with tracheid diameter. This suggests a possible reason why tracheids with a diameter above around 100 microm have failed to evolve. This possibility has been investigated by obtaining an estimate for the resistance of a single pit, and incorporating it into analytical models of tracheid resistance and wood resistivity. The hydrodynamic resistance of the bordered pits of Tsuga canadensis was investigated using large-scale physical models. The importance of individual components of the pit were investigated by comparing the resistance of models with different pore sizes in their pit membrane, and with or without the torus and border. The estimate for the resistance of a real bordered pit was 1.70x10(15) Pa s m(-3). Resistance of pits varied with morphology as might be predicted; the resistance was inversely proportional to the pore size to the power of 0.715; removing the torus reduced resistance by 28%, while removal of the torus and border together reduced it by 72%. It was estimated that in a 'typical tracheid' pit resistance should account for 29% of the total. Incorporating the results into the model for the resistivity of wood showed that resistivity should fall as tracheid diameter increases. However, to minimize resistance wider tracheids would also need to be proportionally much longer. It is suggested that the diameter of tracheids in conifers is limited by upper limits to cell length or cell volume. This limitation is avoided by angiosperms because they can digest away the ends of their cells to produce long, wide vessels composed of many short cells.
先前对植物茎干流体动力学的研究表明，通过管胞侧壁具缘纹孔的流动阻力占其总阻力的很大比例，且该比例随管胞直径的增加而增大。这提示了直径超过约100微米的管胞未能进化的一个可能原因。通过估算单个纹孔的阻力，并将其纳入管胞阻力和木材电阻率的分析模型中，对这一可能性进行了研究。利用大规模物理模型研究了加拿大铁杉具缘纹孔的流体动力学阻力。通过比较纹孔膜孔径不同、有无 torus 和纹孔缘的模型的阻力，研究了纹孔各个组成部分的重要性。一个真实具缘纹孔的阻力估计值为1.70×10¹⁵ Pa·s·m⁻³。纹孔的阻力随形态变化，正如所预测的那样；阻力与孔径的0.715次方成反比；去除 torus 可使阻力降低28%，而同时去除 torus 和纹孔缘可使阻力降低72%。据估计，在一个“典型管胞”中，纹孔阻力应占总阻力的29%。将结果纳入木材电阻率模型表明，电阻率应随管胞直径的增加而下降。然而，为了使阻力最小化，更宽的管胞也需要按比例长得多。有人认为，针叶树中管胞的直径受到细胞长度或细胞体积上限的限制。被子植物避免了这种限制，因为它们可以消化细胞末端，从而产生由许多短细胞组成的长而宽的导管。
