THE CENTS EQUISETUM, 



107 



may become infolded between the bundles and breaking in the bottom of the sinuosities, 

 may unite round the individual collateral strands. The pith by this process is supposed 

 to become continuous with the fundamental tissues of the outside cortex. This type of 

 axis is denominated astelic and is said to be characteristic of the Equisetaceae, the Ophio- 

 glossaceae, and many limicolous phaenogamous orders e. g>, Ranunculaceae, Nymph- 

 aeaceae, Alismaceae, etc. This account by no means exhausts Van Tieghem's types, but is 

 sufficient for the present purpose. 



Strasburger (Histologische beitrage 3) in a recent comprehensive investigation of the 

 anatomy of vascular plants, adopts Van Tieghem's views except in the case of the so-called 

 astelic t \ pe, which he prefers to consider as monostelic, regarding - the individual ecto- 

 dermal sheaths, which surround the bundles in this type, as not morphologically equivalent 

 to the general endodermis, surrounding the complex of bundles in monostelic axes. This 

 view is the result of two considerations; first, he is of the opinion that it is difficult to con- 

 ceive that the pith of different species of the same genus should lie sometimes within and 

 sometimes without the stele, as for example in the case of the Equisetaceae and Ranun- 

 culaceae; and, secondly, since he attributes a great physiological importance to the pith, 

 in connection with the conductive functions of the central cylinder, he thinks it impossible 

 to consider it as in any case separated from the bundles. 



His fust objection had really been anticipated by Van Tieghem (Journ. de hot., 1890, 

 p. 365), however, in the case of the Equisetaceae. Van Tieghem there shows that the stem 

 of the Equisetaceae is always astelic in the adult plant and that the pith is consequently 

 always continuous with the cortex. Van Tieghem's theory however would scarcely allow 

 of this explanation in the case of the Ranunculaceae, but it is not necessary to consider 

 that group in the present connection. In the Equisetaceae, he has shown from Pfitzer's 

 researches and his own, that the individual endodermal sheaths of the bundles and the 

 internal general endodermis, where they occur, are at intervals continuous with the outer 

 endodermis, and as Strasburger admits the morphological value of the outer endodermal 

 layer, he must necessarily grant the same value to its inward extensions. In regard to 

 his objection to separating the pith from the bundles very little need be said. The former 

 is frequently absent or composed of dead cells, especially in perennial plants, so that in 

 general it can scarcely have any very important conductive functions. Zenetti (Leitungs- 

 system von Osmunda regalis Bot. zeit., 1895) has moreover recently shown that in 

 Osmunda regalis where the presence of a distinct endodermis surrounding the leaf-bundles, 

 makes it easy to decide, there are no strands of medullary tissue, accompanying the leaf- 

 traces on their upward course, as Strasburger has suggested in connection with the 

 important conductive functions, which he lias assigned to the pith. We may accordingly 

 conclude that Van Tieghem's conception of astely, in the case of the Equisetaceae at least, 

 is not open to the objections suggested by Strasburger. 



