arising through differentiation of the same crust. In the new overgrowing crusts, 
the limiting lines between the successive zones of tissue are more or less inclined. 
This complex structure is not always found even in old fronds. Crusts up to 1,5 mm 
in thickness showing only one hypothallium are frequently met with, 
The muciferous canals of the sporangial conceptacles open outwards in a low 
hollow surrounded by a slightly elevated border, but it is sometimes very slight or 
Fig. 146. 
Lithothamnion polymorphum, vertical sections of sporangial conceptacles, A, the first division of the sporangium 
is not accomplished, the following not yet begun. B, with a very long sporangium. C, conceplacle with undivided 
and fully divided sporangia. D, ripe sporangium. E, empty sporangial conceptacle with covering tissue; below, 
limiting line of zones and outline of empty conceptacle filled wilh regenerative tissue. A—D 200:1; E 63:1. 
scarcely perceivable, if at all (fig. 146). The sporangia are at least divided into 4 
spores. As stated by FosLiE, the sporangia are 80—110 long, 25—45, broad; the 
outermost ones, however, reach a greater length (fig. 146). After the evacuation of 
the sporangia, the conceptacles become sunk in the crust by continuation of the 
growth in thickness of the frond, but their surroundings may then behave in 
different manners. 1) The filaments of the roof grow upwards in accordance with 
those of the surrounding frond, and the conceptacle forms an empty round hole. 
2) The roof falls into decay and the conceptacle is filled more or less completely 
by tissue growing inwards from the tissue which is developed by increase in thick- 
ness of the surrounding part of the frond and united over the conceptacle. 3) The 
conceptacle is completely filled by a tissue produced from the bottom of the con- 
