SPHENOPSIDA 107 



some twenty-five species are known. Eleven of them occur 

 in the British Isles, where they are known as 'horse-tails'. 

 The genus is distributed throughout the world with the 

 exception of Australia and New Zealand, from which 

 countries it is completely absent. All the species are her- 

 baceous perennials, but there is an interesting range of 

 growth habits, for some are evergreen, while others die back 

 to the ground each year. Early statements that a Hmited 

 amount of secondary thickening occurs are now discredited, 

 for there is no evidence that a cambium is present in any 

 species. Most species are, therefore, very hmited in size; the 

 largest species, E. giganteum, has stems up to 13 m long, but 

 since they are only 2 cm thick the plant depends on the sur- 

 rounding vegetation for its support. The largest British 

 species, E. telmateia, sometimes attains a height of 2m and is 

 free-standing in sheltered locahties, but most species are 

 much smaller than this and are between 10 and 60 cm tall. 

 In all species there is a horizontal rhizome from which 

 arise aerial stems that branch profusely in some species 

 (e.g. Equisetum telmateia, E. arvense) or remain quite un- 

 branched in others (e.g. E. hiemale). The leaves, in all species, 

 are very small and are fused into a sheath, except for their 

 extreme tips which form teeth round the margin of the 

 sheath. They are usually without chlorophyll, photosyn- 

 thesis being carried out entirely by the green stems. In the 

 past, there have been discussions as to whether the small 

 leaves of Equisetum represent a primitive or a derived con- 

 dition, but, in the hght of the fossil record, it is now clear 

 that they have been reduced from larger dichotomous struc- 

 tures (i.e. that they are derived). The stems are ridged, each 

 ridge corresponding to a leaf in the node above, and the 

 ridges in successive internodes alternate with one another 

 (as, of course, do the leaves in successive leaf-sheaths). 

 There are, however, some departures from this regular 

 alternation, as the number of leaves in a whorl diminishes 

 from the base to the apex of the stem.^^ 



