150 MANUAL OP BOTANY 



Class VII.— EQUISETIN^. 



The plants included in this class, which only includes the genus 

 Equisetum, are characterised by a peculiar habit ; they gene- 

 rally have a much-branched subterranean rhizome, from which 

 sub-aerial shoots are given off which rise erect and may attain a 

 height of six or seven feet. These aerial shoots are of two kinds, 

 one purely vegetative and the other ultimately becoming termi- 

 nated by a cone-like collection of sporophylls, forming a flower. 

 The shoots are surrounded at every node by a ring of small scale- 

 like leaves cohering together at their bases, in the axil of each 

 of which a branch is produced, causing a succession of whorls of 

 branches to appear. Each of these has the same structure as 

 the stem from which it arises (fig. 909). 



The roots are adventitious and are produced from the nodes 

 of both rhizome and sub-aerral stem. In the former case they 

 grow into elongated structures, but in the latter they remain 

 rudimentary and never grow out from the tissue of the stem. 



In a few species the flowering shoot is different from the 

 form described above ; it either does not produce branches, or 

 very few occur upon it, or they are not developed till the spores 

 are shed. In one or two oases the sub-aerial shoot, whether 

 sterile or fertile, remains almost or quite unbranohed. 



The green colour of the plant is due to the stem, the leaves 

 being brown and scaly. 



The sporophylls are gathered together at the apex of the 

 fertUe shoot, and they form a conical mass which may pi'operly 

 be considered as a flower. Its structure is a thin cylindrical 

 axis on which a number of peltate leaves are arranged close 

 together in a succession of whorls. Each leaf or sporophyll 

 consists of a thick flattened head, to the centre of which a stalk is 

 attached at right angles. On that side of the peltate head or 

 blade which is turned towards the axis, there are a number of 

 sessile sporangia. Each leaf may bear from five to ten, all of which 

 contain numerous spores {fig. 908). 



Just below the flower there is a ring of curiously modified 

 leaves, forming what is known as the armulms. 



Equisetum grows by means of an apical cell, as do the ferns. 

 Such a cell is to be found at the apex of stem, branch, and 

 root (fig. 910). 



