ICS 
CHAPTER XXV. 
This race erf plants bears an aspect altogether different 
that of the foregoing groups; and indeed they have no 
- obv i 0 ^ affinity with any existing order -of plants. In 
^ ^ ro ' v * b have a certain resenhblance to Hie 5 
JSphearas and Casuarinas, but this resemblance is confined 
' to them general aspect. With Fems and Club-mosses they 
have little in common. Their most direct relationship is 
the aquatic group Chora. 
_ The Horsetails are distinguished from other plants by the 1 
following characteristics. They are leafless, branching, with 
hollow jointed stems, separable at certain joints, which occur 
at intervals where they are solid, and surrounded by mem- ; 
branous toothed sheaths: each length, in fact, terminates ; 
sheaths, into which the base of the 
next length fits. The sheaths seem'to represent abortive I 
leaves. The fructification consists of terminal cone-like 
heads. 
_The stems consist chiefly of cellular matter, coated exter¬ 
nally by a layer of hard woody tubes, from which plates of 
asmiiiar nature project towards the central cavity. Between 
the outer and inner surface of this cylinder-like stem, occur- 
2}^“? re oj tubes, or air-cavities, differing in size 
and position ; these afford, by their comparative sizeTnumber, 
“P^&ement, excellent auxiliary marks for the recogni- 
faon of the species. The cuticle or skin abounds in silicious 
particles secreted in the form of little warts, which impart 
to the surface a greater or less degree of roughness in pro-; 
porhoii to their prominence. In some species this deposit 
rfahaons matter is so great, that the whole of the vegetable 
auer is so great, that the whole ^ 
y be destroyed by maceration, the form of tne 
•reserved entire in the flinty coating. It has 
that the ashes contain half their weight of 
rtion of the cuticle to the analysis of 
a high magnifying power, Dr. Brewster 
irrangement of the silicious particles, 
• distributed in two lines paraue to the 
T” oucm Y extending over the whole surface. 
Jhe greater number of the particles were seen to form 
ample straight lines, but the rest were grouped into oval 
