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 around 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- Wa 
ing and successively dividing until | “fe 
a flattish. irregular gametophyte as 
(the prothallium) a few milli- \/ 
meters in breadth is produced. It *~ 
bears antherids and archegones (, Mo 199— Raveehytes). 
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 
