358 BOTANICAL GAZETTE. 
a 
vides still further by walls parallel to the first. The segment 
thus grows faster in length than in breadth, and is rapidly pushed 
out beyond the apical cell, which thus gradually comes to lie at 
the first, and from this time in the growth can no longer be traced 
back to one cell, 
About this time, or sometimes before the obliteration of the 
apical cell, the cells in its vicinity, which had hitherto only di- 
vided in two planes, so as to form a single layer of cells, are now 
divided by walls parallel to the surface of the prothallium, form- 
ing the beginning of the cushion of tissue that occupies the base 
of the notch in the front of the prothallium ; where this is begun 
early, the subsequent growth of the prothallium results in the 
formation of a thickened rib running through the middle for 
nearly its whole length. 
he prothallium is fastened to the ground by numerous root- 
hairs that arise principally from the cells of the lower part, as 
A nt : + Sani 
1 many species, e. g. Aspidium spinulosum, Cystopteris fragil 4 
: Onoclea sensibilis, there are developed small papillee from the 
indeed, probably none of them are absolutely inflexible, am 
