of the Anatomy of the Genus Selaginella , Spr. 1 8 1 
ridge ; in II a condition similar to that seen in the adult stem 
of 5. uncinata is reached ; and finally in I a single, almost 
cylindrical, xylem-mass with four marginal protoxylems 
appears. Still earlier conditions can only be obtained by 
examination of embryonic primary shoots, material for the 
study of which I have not in sufficient quantity to enable me 
at present to illustrate thoroughly. 
The histological peculiarities of the stem may next be 
considered. In the erect axis there is a small-celled epidermis, 
as usual cuticularized, several layers of stereome. and a large- 
celled cortex, the cells of which become smaller towards the 
lacunae. There are in fully developed stems three lacunae, 
the cortical walls of which bear a well-marked siliceous deposit. 
The trabeculae are short, and the endodermal cell is either 
connected directly with the cortical cells or with long creeping 
multicellular filaments enclosed in a siliceous deposit. A 
siliceous deposit also occurs between the double steles in old 
stems. The pericycle is from one to four layers thick. There 
are a few crushed protophloem elements. The sieve-tubes 
surround the xylem, but are only one layer deep opposite the 
protoxylems, and may be interrupted by parenchyma oppo- 
site the protoxylems of the median stele. One to two layers 
of parenchyma separate the sieve-tubes from the xylem. 
In the single stele of the primary creeping axis the peri- 
cycle is not markedly differentiated from the inner cortex, 
and cuticularized endodermal cells like those present in the 
erect-axis are entirely wanting. In the small intercellular 
spaces between the innermost layers of the cortex a minute 
deposit of silica appears. The sieve-tubes are quite absent 
opposite the marginal protoxylems (PL XI, Fig. 68). Where 
the steles separate from each other — as, for example, in situa- 
tions represented at Figs. 69 and 70— the trabeculae are in 
the form of special cells of the pericycle, in no particular 
differing from those which enclose the steles, save that they are 
cuticularized entirely like those which form the outermost layer 
of the pericycles themselves. I think this is a point of some 
importance, as supporting the common origin of the pericycle 
