EQUISETACEyE 109 



sieve-tubes and narrow cambiform cells. In most species {e.g. E. ar- 

 vense) a general bundle-sheath, or plerome-sheath, consisting of a single 

 layer of cells, encloses the entire circle of bundles, as in most Flowering 

 Plants ; while in others (E. limosum, L., and littorale, Kuhlw.) each 

 individual bundle is enclosed in a separate special bundle-sheath, as in 

 ferns. In the colourless fertile shoots they bend out into the pedicels of 

 the peltate scales of the ' fructification,' as they do into the leaf-sheaths. 



The growth of the stem takes place through the activity of a single 

 large pyramidal apical cell with an inverted triangular base. There is 

 no other group of plants which exhibits such a well-defined single apical 

 cell or exclusively apical growth. Normally the terminal bud never 

 branches, branching taking place solely by lateral buds produced at 

 the nodes. It was formerly thought that the Equisetaceae display the 

 only known example of lateral branching being due to the formation of 

 endogenous buds ; but recent researches have shown (in E. arvense) 

 that these lateral bud^ are not of endogenous origin, but originate from 

 a single superficial cell of the growing point of the stem in the ordinary 

 way. The segments resulting from the first divisions of the apical cell 

 lie in three straight rows, and are arranged in a spiral divergence of one- 

 third. 



The ?'oots are produced in whorls at the nodes of the underground 

 stem, in direct connection with lateral buds, or, under favourable con- 

 ditions, at the nodes of the aerial stems. They are furnished with a 

 root-cap, increase by the segmentation of a single apical cell, and are 

 penetrated by an axial ' vascular ' bundle surrounded by a large air- 

 cavity formed by the coalescence of intercellular spaces owing to the 

 absorption of intermediate cells. The weak bundle, in which the 

 tracheides are but feebly developed, is concentric, with the xylem in the 

 centre. The secondary or lateral roots, which arise in acropetal succes- 

 sion on the primary root, differ in their origin from those of ferns and 

 other Avascular Cryptogams. In these latter it is the innermost layer of 

 cells belonging to the fundamental cortical tissue immediately surround- 

 ing the axial ' vascular ' bundle that becomes differentiated into the 

  bundle-sheath or endodermal layer, within which lies the pericambium 

 of the bundle itself ; and the lateral roots originate from the innermost 

 layer of the cortex separated by the pericambium from the bundle. In 

 the roots of Equisetaceae the pericambium is wanting, and its place is 

 to a certain extent supplied by the innermost cortical layer, from which 

 the lateral roots spring, and therefore in close contact with the periphery 

 of the axial bundle. The bundle-sheath itself is, in the Equisetacese, 

 formed from the row of cortical cells next to the innermost row, and not 

 from the mnermost row itself, as in other Vascular Cryptogams. 



