96 THE BOOK OF NATURE STUDY 



spore, and as the spores dry the threads straighten out with a 

 jerky and wriggling movement. In this way the threads cause 

 the mass of spores to be spread out and more easily blown away 

 by the wind. 



As soon as the spores are shed the fertile shoot dies down, 

 its work being done, and meanwhile the ordinary sterile shoots 

 come up from the soil. The fertile and sterile shoots arise in- 

 dependently from the underground stem, which is jointed and 

 bears sheath-collars like the erect shoots. The creeping stem 

 bears, besides the collars, (i) roots, (2) erect fertile and sterile 

 shoots which come above ground, and (3) short swollen branches 

 tubers which contain starch and are able to produce new 

 plants. 



The spores are green, and will only germinate if sown immedi- 

 ately when ripe. The growth of the prothalli, however, is very 

 slow, except just at first. Small male prothalli, bearing sperm 

 pockets, are obtained within a month, but it is rather difficult 

 to prevent " damping off " (due to fungi), and to get the larger 

 female prothalli which eventually bear the young horsetail 

 plants. 



There are several other horsetails in Britain which resemble 

 our type in all essential points, but differ in details. The Wood 

 Horsetail (E. sylvaticum), which grows in damp woods and shaded 

 places, is the most graceful kind. Its slender stem, about a foot 

 high, bears collars with broad teeth, and the drooping branches 

 (about a dozen in each circle) bear secondary branches, usually 

 in pairs, which, with the drooping of the primary branches, give 

 the plant an elegant tufted appearance. The profuse branching, 

 as compared with other horsetails, is evidently connected with 

 the different habitat, the wood horsetail in this way presents 

 a large amount of green surface, to catch as much as possible of 

 the scanty light falling upon it in its shaded home. The Giant 

 Horsetail (E. maximum) resembles the field horsetail pretty 

 closely, but is much larger, its barren stems being sometimes 

 six feet high, and bearing crowded circles of long branches 

 about thirty in each circle. The stem is white and feels quite 

 smooth, the ridges and furrows being only slightly developed, 

 and is about half an inch in diameter. It grows chiefly in bogs, 



