524 BOTANY OF THE LIVING PLANT 



cortex. Centrally there is a large pith-cavity. Round this vascular strands 

 are disposed in a regular cylinder, their number corresponding to that of the 

 leaves in each whorl, and with details related to the moist habitat of these 

 plants. There is a near analogy with the structure of certain seed-plants, 

 but there is no persistent cambium in the living species of Equisetum : though 

 in the Calamarian fossils, while the primary scheme is the same, an active 

 ring of cambium led to the development of a massive woody trunk. 



The sporangia are large, and each contains numerous homosporous spores. 

 These germinate and produce autotrophic, dorsiventral prothalli, bearing 

 numerous erect lobes. The gametangia are essentially of the Fern type : but 

 the embryo appears as a spindle-shaped body, the initial cell at its apex being 

 defined by the first segmentations. It is difficult to bring these features 

 into any near relation either with those of Ferns or of Lycopods. Perhaps 

 the nearest comparison among living plants would be with Tmesipteris. 

 But the facts of segmentation of the zygote bear a special value, since 

 Equisetum is the only genus of the ancient Articulate Plants in which the 

 embryology is known. The Horse-tails appear as an isolated type in modern 

 vegetation, and present a phyletic problem of their own. 



A more full description of the Equisetales will be found in Primitive Land 

 Plants, Chapter X. 



