xii EQUISETINEM 481 



turn and E. scirpoides (Fig. 281, B), the aerial branches are all 

 similar and often are quite unbranched. The foliar sheaths 

 show considerable variation. In the fertile stems of E. tel- 

 mateia (Fig. 266) they are extremely large and the ribs very 

 prominent, but the separate leaves are not all distinct at the 

 apex, but the sheath splits into a few very deeply cleft pointed 

 lobes. In the sterile shoots, however, and in all the stems of 

 most species, the teeth are very distinct and the foliar sheath 

 much shorter. The number of teeth varies from three in 

 E. scirpoides, to thirty or forty, or even more, in E. telmateia 

 and E. robustum. In E. silvaticum the branches produce 

 whorls of secondary branchlets. 



Sadebeck (8) recognises 24 species of Equisetum. The 

 largest forms occur in tropical America, where some species, 

 e. g., E. giganteum, reach a height of 3 to 12 metres, but are 

 relatively slender, the stem usually not exceeding two or three 

 centimetres in diameter, and requiring support from the shrubs 

 and trees among which it grows. E. Schaffneri is described as 

 having a stem about two metres in height with a thickness of 

 10 centimetres, but with a very large central cavity, so that it 

 is not very strong. In some of the larger species, e. g., E. gi- 

 ganteum, cones may be borne at the end of the lateral branches, 

 as well as at the apex of the main shoot. 



Fossil Equisetineoz 



The living genus Equisetum is represented in a fossil condi- 

 dition by a number of closely allied forms, perhaps generically 

 identical, and usually united under the name Equisetites. Be- 

 sides these, there are several types differing materially from 

 Equisetum, but nevertheless undoubtedly related to the living 

 forms. The most important of these fossil forms are the char- 

 acteristic Palaeozoic fossils belonging to the Calamitacese and 

 Sphenophyllacese. A further discussion of these forms will 

 be left for a later chapter. 



Affinities of the Equisetinecz 



The Equisetinese, as will be seen from the account of the 

 fossil forms, are a very ancient group, and their relation to the 

 other Pteridophytes somewhat problematical. The modern 

 31 



