504 Uphof. — Contributions towards a Knowledge of 
xylem (the vascular bundles of a stem have more), and is strongest 
developed in the oldest part of the aerial root of Wildenowi. The 
-tracheides of the metaxylem differ greatly in number in the various species ; 
they are abundant in 5 . Wildenowi and >S. rubella , while only a few are 
to be found in .S'. serpens . The types of tracheides are the same as in 
the stem. 
The author observed that a root-cap is present on all roots which are 
in possession of root-hairs, whether these roots have been developed in the 
air or in the soil. On the other hand, roots which do not have such root- 
hairs, as is the case with the so-called rhizophores, are correlated with the 
absence of a root-cap. As the writer demonstrated, some species may lose 
the root-cap when terrestrial roots are forced to develop in the air ; on the 
other hand, the author was unable to force plants which develop roots with 
root-hairs and root-cap in the air, such as S. grandis , to form in some way 
or other roots without root-hairs or root-cap ; although 5 . grandis may 
form in a dry environment aerial roots without root-hairs. 
The root-cap and root-tip are very simple in construction, and the 
development has already been studied by Treub ( 14 ), van Tieghem ( 13 ), 
and other investigators. The root-cap is pointed in .S'. grandis ; in 
Bakeriana , A. rubella , and A. Wildenowi it is rounded, and is especially 
well developed in the latter species. In homophyllous species, e. g. S. ru- 
pestris , S. rupicola , and A. densa, it is least developed and is composed of 
but a few cells. 
Before giving a description of the anatomy of roots of various species 
of Selaginella the author wishes to state that he has made no study of this 
organ in the young sporophyte. An account of the development and com- 
parative study of the latter will be given in another publication. 
Selaginella Wildenowi. 
The aerial roots or rhizophores are in this species strongest developed of 
all (Fig. I, 1). They attain a length of 90 to 260 millimetres, the diameter 
ranging from 1 to about 2*5 millimetres. They are red in colour, the 
pigment being dissolved in the vacuoles of the cells, especially those of 
the hypodermis and cortex. They may be developed at any distance up 
the plant. 
The epidermis is composed of smaller cells than the hypodermis and 
soon becomes thickened by secondary layers. The hypodermis is com- 
posed of three to four layers of cells. The following cells of the cortex are 
considerably larger than those of the former tissues, and are thin-walled and 
usually composed of twenty to twenty-five rows of cells in the old rhizo- 
phores or about fifteen to twenty rows in those of the first branching (Fig. 
VI. 1-5, and Fig. VII). The endodermis is well developed in the main 
rhizophore, the cell-walls in young specimens showing a varying degree of 
