HORSETAILS 263 



elongated. The stomata which are present in all the 

 chlorophyll-bearing parts of the plant, are arranged with 

 more or less regularity in longitudinal rows; on the stem 

 they occur in the channels between the numerous ridges. 

 The vascular bundles of the stem are disposed in a cyl- 

 inder and run parallel with each other from node to 

 node, where they join with one another. They contain 

 tracheary, sieve and fibrous tissues, arranged somewhat 

 as they are in the bundles of flowering plants. 



471. The spores of Horsetails are produced in cones at 

 the summit of the stems. The cones are composed of 

 crowded whorls of shield-shaped leaves (sporophylls), 

 each of which bears upon its under surface five to ten 

 sporangia. The spores are spherical, and at maturity 

 the outer wall splits spirally into four narrow filaments 

 {elaters) which unroll when dry, and roll up aroimd the 

 spore again when moistened. Their office seems to be 

 to aid in setting the spores free from the spore-cases. The 

 spores germinate soon after falling 

 upon water or moist earth, enlarg- 

 ing and successively dividing until 

 a flattish irregular gametophyte 

 (the prothallium) a few milli- 

 meters in breadth is produced. It 

 bears antherids and archegones (^^^^ l^-^^ZX^osy 

 resembling those of the ferns upon 



its lobes or their edges; in some cases both sexual organs 

 are on the same gametophyte, while very commonly 

 they are upon separate gametophytes, although the 

 spores show no differences. The sperms are spiral and 

 multiciliated. 



472. This class contains but one family {Equise- 

 taceae), including a single genus,' Equisetum, and twenty- 

 four species of herbaceous plants usually a meter or less 



