272 THIRD GROUP. VASCULAR CRYPTOGAMS. 



of a tooth of the sheath from below upwards, while the other is formed in the internode 

 itself from above downwards; the formation of vessels begins in both at the angle where 

 the two arms meet, and advances in opposite directions; the lower extremity of each 

 bundle joins by two lateral commissures with the two next bundles, which alternate with 

 it, of the next internode below; thus the Equisetaceae have only ' common ' bundles. 

 The vascular bundles seen in transverse section resemble the bundles in the Monocoty- 

 ledons and especially in the grasses ; the annular, spiral and reticulated vessels which 

 are first formed and belong to the axile side of the bundle, and the thin-walled cells 

 between them, are subsequently destroyed and in their place is a canal (lacuna) which 

 runs through the length of the bundle on its axile side ; right and left of this 

 canal towards the outside, are a few not very broad reticulately thickened vessels; 

 radially outside these and in front of the canal is the phloem, consisting of a few broad 

 sieve-tubes and narrow cambiform cells, and towards the circumference of some narrow 

 thick- walled bast-like cells. In some cases the separate bundles are surrounded by 

 bundle-sheaths, as in E. limosum, but it is more usual to find a common plerome-sheath 

 of one layer of cells running round the whole circle of bundles on the outside, as in 

 most Phanerogams. 



Appendix. Numerous species 1 of fossil plants, which appear from their structure 

 to belong to the Equisetaceae, have been found in very various formations from the 

 Lower Trias to the Tertiaries. They occur sometimes in vast quantities, as Equisetum 

 arenaceum for instance in the sandstones of the Upper Trias. The stems are stated to 

 vary from four to twelve centimetres in diameter, the sporangiferous spikes to be two 

 and a-half centimetres and upwards in diameter, while the stems are supposed to have 

 reached a height of from eight to ten metres. 



The CALAMITES 2 are Equisetaceae which appear in the older geological formations, 

 beginning in the carboniferous limestone, culminating in the coal-measures and dis- 

 appearing in the Permian formation. The spikes of sporangia are either not known, or 

 so badly preserved (Calamostachys] that their structure cannot be determined ; it 

 remains doubtful therefore whether they were homosporous or heterosporous forms. 

 The stems had neither leaves nor leaf-sheaths, or else these were very transitory forma- 

 tions and soon fell off ; in other respects the structure of the stems resembles that of the 

 Equisetaceae ; their surface was marked with ridges, and they had a central hollow 

 divided by diaphragms. 



B. HETEROSPOROUS EQUISETINEAE. 



These are all fossil species forming the group of the Annularieae, with which it is 

 probable that the ' Asterophyllites ' should be associated. 



I. ANNULARIEAE 3 . The stem of the Annularieae was as much as eighty centimetres 

 in diameter, with feebly developed dermal and vascular systems. There were dia- 



1 Twenty, according to Renault's computation in his Cours de botanique fossile, II. Bd. 1882. 



2 Excluding Calamodendron, Arthropitys, Calamites gigas, etc., the connection of which with 

 the Calamites is at present at least doubtful. [The views of Prof. Williamson and other British 

 Palaeophytologists regarding the structure and affinities of the Calamites are quoted at some length 

 by Vines on p. 40 of the second edition of the English translation of Sachs' Lehrbuch, and references 

 to literature are there given. The valuable monographs of Prof. Williamson ' On the Organisation 

 of the Plants of the Coal-measures' which have appeared in the Phil. Trans, at intervals from 1871 

 to the present time should be also consulted.] 



3 Renault, loc. cit. p. 126 ff. Compare Schenk, Ueber Fruchtstande fossiler Equiseten (Bot. 

 Ztg. 1876). The short description given in the text from Renault may serve at least to draw atten- 

 tion to these interesting types, in which there is much that is yet uncertain. We cannot enter 

 further here into disputed or doubtful points. [See Prof. Williamson in Phil. Trans. 1874.] 



