a aon eT. Is ae 
1901] SPORANGIA AND GAMETOPHYTES OF SELAGINELLA E27 
favorable conditions of moisture, have rooted, and later have 
severed their connection with the parent plant. In examining 
many colonies, very few young plants that originate from spores 
are in evidence, although the soil may be thickly beset with the 
spores that have been shed. Bits of the old plants that have 
been torn off by the action of the wind or rain are frequently 
caught in crevices along the precipices, and it is from these that 
new clumps are most frequently started. 
In fig. 124 I have endeavored to convey an idea of the struc- 
tural adaptation of the strobilus to its austere environment. The 
closely overlapping sporophylls form four rows, in whose axils 
the sporangia have little space to develop. The growing apex 
is protected by at least twelve and frequently sixteen sporo- 
phylls which envelop it. The epidermis is two or three layers 
of cells thick on both surfaces of the sporophyll, except in a 
shallow groove running lengthwise along the middle of the 
ventral surface. In this groove are the comparatively large 
crowded stomata, which are protected by the next older over- 
lapping sporophyll of the same row. While the leaves are yet 
very small the apex ceases to grow and becomes transformed 
into a branched spine.* The formation of the horny epidermis 
proceeds from the apex toward the base. Between the ligule 
and the sporangium the tissue retains its meristematic nature. 
The vascular bundle occupies the central shaft of the strobilus, 
and is augmented from the apex downward by the leaf traces 
that join it from the sporophylls. Four large communicating air 
Spaces, spanned by trabeculae, surround the entire system. 
These communicate with the exterior by certain larger chambers 
into which the stomata open. There is little closely compacted 
tissue in either sporophyll or stem. All the cells except the 
epidermis and the vessels contain chlorophyll. This structure 
is in marked contrast to the delicate unprotected sporophylls of 
S. apus. 
A single superficial cell, which uniformly is so close to the 
base of the subtending sporophyll that it is impossible to deter- 
mine whether it belongs to the stem or the leaf, is the origin of 
