The medullosan pteridosperms
The medullosan pteridosperms
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髓蕨植物
DOI：
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发表时间：
2008
期刊：
影响因子：
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通讯作者：
T. Delevoryas
中科院分区：
文献类型：
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作者：
Wilson N. Stewart;T. Delevoryas
在蕨类植物门（苏铁目）的成员中，髓果属在古生代植物区系的文献中占有重要地位。该茎属归属于髓科（也包括茎属Sutcliffia和Colpoxylon），其典型特征是存在几个贯穿茎的中柱轴（图1）。这一特征将髓蕨科与古生代蕨类植物的另外两个科--Lyginopteridaceae和Calamopityaceae区别开来。每个维管轴（以下称为“中柱”，关于其形态的注释保留在后面的页面）由初生木质部的中心体组成，被次生木材，形成层和韧皮部包围。初生木质部在本属所有代表植物中结构相当恒定，由散布在薄壁细胞中的大管胞组成。通常，这些管胞垂直延伸，但在欧洲二叠纪的一些茎类型中发现了一种变化，其中初生管胞几乎总是采取水平路线（Rudolph，1922）。次生木质部也由大的管胞组成，管胞中放射出高的射线。次生组织的发育程度在不同种类的髓花属植物中并不总是相同的，并且通常在茎的中心比在中柱的外表面上发现更多的次生发育。Schopf（1939 b）将这种情况称为“内心”，以将其与“外心”配置区分开来，后者的情况相反。韧皮部保存下来后，
Among the members of the Pteridospermae (Cycadofilicales), the genus Medullosa is one that has figured prominently in the literature concerned with Paleozoic floras. That stem genus, assigned to the family Medullosaceae (which also includes the stem genera Sutcliffia and Colpoxylon) is typified by the presence of several stelar axes running through the stem (Fig. 1). This feature distinguishes the Medullosaceae from the Lyginopteridaceae and the Calamopityaceae, the other two families of Paleozoic pteridosperms. Each of the vascular axes (referred to subsequently as "steles ", with notes concerning their morphology reserved for a later page) consists of a central body of primary xylem, surrounded by secondary wood, a cambium and phloem. The primary xylem is rather constant in structure among all the representatives of the genus, consisting of large tracheids interspersed among parenchyma cells. As a rule, these tracheids extend vertically, but a variation is to be found among some of the stem types of the European Permian where the primary tracheids almost always assume a horizontal course (Rudolph, 1922). Secondary xylem is composed also of large tracheids, among which radiate tall rays. The extent of development of secondary tissues is not always the same in the various species of Medullosa, and often more secondary development is to be found extending toward the center of the stem than on the outer face of the stele. Such a condition was termed " endocentric" by Schopf (1939b) to differentiate it from an " exocentric " configuration where the reverse holds true. Phloem, when preserved, can be seen to