368 EQUISETALES 
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 
Subsequently the type 
became less prevalent, till at 
the present day it is repre 
sented only by the cosmo- 
politan genus Eguisecum, 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 Lguisetum itself. 
size. 
EXTERNAL CHARACTERS. 
It will be unnecessary to 
describe the characters of the 
shoot in Zguzsetum 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 
Fic. 193. 
Equisetum maximum, Link, Left half of a radial longi- 
tudinal section below the apex of an underground bud (in 
September). wv, lower part of the apical cone; 3’, 4”, b'”= 
leaves; m=pith; v, v=meristematic ring; g, g=cell-layer 
features, for beneath them all 
lies a general unity of plan 
which is closely followed, 
whether the shoot be under- 
from which the bundles of the leaf-teeth arise ; 7, 7=the first 
beginning of a branch. (After Sachs, from Engler and 
Prantl, Vat. Pflanzenfam.) ground 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 
