416 



PART III. THE CLASSIFICATION OF PLANTS. 



CLASS VI. 



This class includes, among existing plants, only the homosporous 

 order, Equisetaceae ; but there are many extinct fossil forms, some 

 of which are undoubtedly heterosporous. 



Order 1. Equisetaceae. This order includes the single genus Equisetum 

 (Horsetail). Of the tweuty species of this genus, about half are British (E. 



arvense, maximum, silva- 

 ticum, palustre, limosum, 

 hyemale, trachyodon, va- 

 riegatum, pratense, litor- 

 ale). 



THE SPOROPHYTE. The 

 stem consists of a horizon- 

 tal, subterranean, much- 

 branched rhizome ; some 

 of the branches come to 

 the surface, and are the 

 sub-aerial shoots. The 

 rhizome and its branches 

 are very distinctly seg- 

 mented into nodes and 

 internodes. At each node 

 is borne a whorl of scale- 

 leaves forming a con- 

 tinuous sheath. The 

 branches, as also the ad- 

 ventitious roots, spring 

 from the nodes, a bud 

 being developed in the 

 axil of each leaf, and an 

 adventitious root from the 

 base of each bud ; but in 

 the sub-aerial shoots the 

 rudimentary roots do not 

 grow out, whilst in the 

 subterranean shoots re- 

 latively few of the buds 

 grow into branches. In 

 certain species (E. arvense, 

 silvaticum, maximum, pa- 

 lustre, etc.) some of the 

 subterranean branches be- 

 come tuberous. 



The sub-aerial shoots 



FIG. 271. Median longitudinal section of the apical por- 

 tion of a vegetative shoot of Equisetum arvense ; pv apical 

 growing-point ; g bud-rudiment ; g'-g'" stages in the de- 

 velopment of lateral buds ; r r' developing adventitious 

 roots on the buds ; m central ground-tissue (not pith) ; 

 vs developing (common) vascular bundle; n nodal 

 diaphragms. ( x 26 : after Strasburger.) 



generally live for one season only, and are herbaceous in texture, but in some 

 species they persist (e.g. E. hiemale, trachyodon, variegatum). They are 



