368 



EOUISETALES 



collectively characterise the group as a definite one. As regards its past 

 history, the evidences of the existence of the Equisetales extend back to 

 the Devonian period, where they already showed a high degree of 

 elaboration. But these plants formed a more conspicuous feature in the 

 Carboniferous Flora, where they attained their maximum development in 



point of numbers as well as in 

 size. Subsequently the type 

 became less prevalent, till at 

 the present day it is repre- 

 sented only by the cosmo- 

 politan genus Equisetum, with 

 its twenty-four species, showing 

 remarkable uniformity of type. 

 The essential characteristics of 

 the living genus will be taken 

 first, as it is susceptible of more 

 complete study than the fossils : 

 these will be worked in on a 

 basis of comparison with what 

 is seen in Equisetitm itself. 



EXTERNAL CHARACTERS. 



It will be unnecessary to 

 describe the characters of the 

 shoot in Equisetum in full 

 detail, or the comparatively 

 slight modifications of it upon 

 which the species are distin- 

 guished : a brief account will 

 suffice to indicate the essential 

 features, for beneath them all 

 lies a general unity of plan 

 which is closely followed, 



whptVipr trip srinnr HP nnHpr 

 WHCtHCr 



g round or exposed to the air 

 (Fig. 192). The axis is plainly 



the dominant feature of the shoot, and it is always of radial construction : 

 it is terminated by a conical apex with well-marked initial cell. Upon the 

 vegetative axis the leaf-sheaths arise laterally, in close acropetal succession : 

 they are webbed from a very early stage, and when mature consist of 

 clearly marked leaf-teeth projecting upwards from the webbed sheath below 

 (Fig. 193). As the developing internodes lengthen by intercalary growth 

 of the bud thus constructed the leaf-sheaths separate, while the internodes 

 themselves are then seen to be marked by flutings corresponding to the 



FIG. 193. 



Equisetum maximum, Link. Left half of a radial longi- 

 tudinal section below the apex of an underground bud (in 

 September). vK, lower part of the apical cone ; b 1 , b" ', b'" = 

 leaves; z = pith; v, z/ = meristematic ring; g; g= cell-layer 

 from which the bundles of the leaf-teeth arise ; /, z"=the first 



ind 



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