EUROPE AND NORTH AMERICA. 9 
of these is to form a compact or dense variety, by the shortening of 
the internodes, and consequent close approximation of the branch 
fascicles; this is well seen in such states as S. Austind var. tmbri- 
catum, S. paprillosum var. confertum, S. cymbifolium var. congestum, 
S. rigidum var. compactum, and S. acutifolium var. arctum. The 
second is that in which the leaves of the branches which are 
normally straight and imbricated, exhibit a greater or less inclina- 
tion to become squarrose; this does not render the plants so 
conspicuous as the last state, and is sometimes so slight as not 
to require indication by a special name ; and even in cases where 
it may be quite evident, as in S. cymétfolium var. squarrosulum, 
we find intermediate forms which completely connect it with the 
type ; nay, sometimes in the same tuft we may observe the leaves 
on plants at the circumference distinctly tending to squarrose, 
while those in the centre of the patch do not exhibit it at all; 
in this way it is quite possible to pick out the links that join 
S. teres to S. sguarrosum. 
In none do we find the range of variability so extensive as 
in S. acutifolium, and considerable experience is needed to iden- 
tify satisfactorily all the forms of this protean plant. On this 
account we must, I think, rely on histological distinctions for the 
essential character of species, as size, colour, direction of leaves, 
habit, and presence or absence of fibres in the hyaline cells of the 
stem leaves, alike fail in this species to afford any stable grounds 
for the establishment of new ones, and the same considerations 
must guide us in dealing with the rest; they will thus stand on 
firm ground, and be more satisfactory to the true botanist than 
the most extensive series of phantom species. 
The structures we must look upon as of most importance in 
affording specific distinctions are— 
1. The number of cell strata composing the cuticle which 
invests the stem of the plant. 
2. The stem leaves, with respect to their form, relation of the 
two kinds of cells, and structure of the apex. 
3. The branch leaves, as to form, involution of margin, apex, 
relative position of the hyaline and chlorophyllose cells, 
and presence or absence of papilla on their internal 
lateral walls, and also the presence and size of the fora- 
mina. The number and direction of the branches in 
each fascicle, and nature of the retort cells of their 
cuticle, are also to be taken notice of. 
