Sturch. — Harvey ella mirabilis (Schmitz & Reinke). 95 
mirabilis the sporophyte consists of a much-branched fila- 
mentous thallus extending throughout the external portion 
of the gametophyte ; and further, that, leaving out of con- 
sideration that portion of the thallus of the gametophyte 
which is inside the host, the sporophyte approaches very 
nearly in dimensions to the gametophyte. Both generations 
consist of a sub-dichotomously branched filamentous thallus. 
In the gametophyte a large portion ramifies among, and is 
parasitic on, the cells of the host-plant ; while the sporophyte 
ramifies among the cells of the gametophyte, and may be said 
to be parasitic upon it. Thus in Harveyella mirabilis the two 
generations approach each other more nearly in vegetative 
characters than they do in possibly any other Floridean type. 
The tetrasporangium is formed from one of the ordinary 
distal cells of a peripheral filament. This cell is divided in 
the usual way by an oblique wall. The cell (Fig. 6, t) cut off 
is the tetrasporangium, while the remaining cell (Fig. 6, j) 
becomes very attenuated, and its only further development 
is that it often divides once more transversely. The tetra- 
sporangium rapidly increases in size, and its contents become 
more granular. These contents, like those of the carpospores, 
never show any trace of red or brown colour, and take up 
Hoffmann’s blue very readily. After the tetrasporangium 
has reached its full size, its contents are divided into two by 
a transverse wall (Fig. 6, a). As a rule the lower half is 
divided by a longitudinal wall first, and then the upper half ; 
or the longitudinal division in both upper and lower halves 
may take place at the same time. Thus a typically cruciate 
tetraspore is produced. Occasionally (as at Fig. 6, c) the 
longitudinal division in the upper part takes place in a plane 
at right-angles to the plane of division in the lower part : 
thus in Fig. 6, c , the other half of the upper part of the 
tetraspore is behind the half shown. This formation of 
tetraspores takes place around the whole periphery of the 
plant, and tetraspores in all stages of development are found 
at the same time ; so that there is a layer, almost a nema- 
thecium, made up of tetraspores, with a few ordinary peripheral 
