EUROPE AND NORTH AMERICA. 



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. Austint var. imbri- 

 catum, S. papillosum var. co7i/ertum, S. cymbifolhim var. congesUitn, 

 S. rigidum var. compactnm, and 6*. acufi folium var. irctum. The 

 second is that in which the leaves of the branches which are 

 normally straight ant' 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 6*. cymbifolium var. sqnarrosulnm, 

 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. squarrosvm. 



In none do we find the range of variability so extensive as 

 in S. acuti/olium, 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 papillae on their internal 

 lateral walls, and also the presence and size of the fora- 

 mina. The number ai.l direction of the branches in 

 each fascicle, and nature of the retort cells of their 

 cuticle, are also to be taken notice of. 



