茎和根几个次生结构的探讨——植物学教材质疑(九)

Discussion of Some Secondary Structures of Stems and Roots Questioning Botany Textbooks(Ⅸ)

一些植物学教材中,被子植物的几个次生结构长期被误解。木材(次生木质部)切向切面上的年轮呈“倒V”形,但几乎所有植物学教材都描述其为“V”形结构。几年生茎的横切面会在形成层外方产生一些“倒三角”形结构,其多被教材当作韧皮射线,实际上这是次生延长的髓射线的外侧段。横切次生生长的根,可观察到“倒三角”形的次生射线,此结构被一些植物学教材忽略或当作韧皮射线。周皮模式图常有错误标注或误绘;分析这些错误,有助于学生更好地理解周皮的起源和发育。笔者设计了两个探究性实验,帮助学生识别次生结构,并且根据结构与功能的关系去判断器官类型。

Misunderstandings about secondary structures of angiosperms are found in some botany textbooks.Firstly,an annual ring in tangential sections of woods(secondary xylems)has a reverse V-shape,whereas the structure is wrongly described as V-shaped in almost all the textbooks.Secondly,in transverse sections of a few years old stems,there are,outside the vascular cambium,some inverted triangle-shaped structures that are usually regarded as phloem rays or neglected in textbooks.In fact,however,they are the extended external parts of pith rays due to secondary growth.Thirdly,in cross sections of second growing roots,there are also inverted triangle shapes that are actually secondary rays,whereas these structures are neglected or recog-nized as a phloem ray in some botany textbooks.Finally,some model figures showing periderm transverse sections are often incorrectly labelled or drawn,which should be analysed to help students better understand the origin and development of periderm.In addition,two exploratory experiments are designed to tell students how to identify secondary structures and determine organ types based on structure-function relationships.