229 



arising llirougii differentiation of the same crusl. In Ihe new overgrowing crusts, 

 the Hmiting lines hetween 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. 



Lithotltainnion polymorphiim, verticiil sections of sporangial conceptacles, ,4. the first division of the sporangium 

 is not accomplished, tlie following not yet begun. B, with a very long sporangium. C, conceptacle 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 with regenerative tissue. .1 — 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- 



