864 DR R. KIDSTON AND PROF. W. H. LANG ON OLD RED SANDSTONE PLANTS 
inner membrane can be traced. The vesicles belonging to the associated fungus are 
seen covering portions of the thick wall. The same specimen seen from the other 
side (that is, looking at the outer surface of the wall from which only the central 
projecting portion is removed) is represented in fig. 52. The groups of vesicles are 
here seen from above, and not in section. The cavity of the large spore was filled 
with a fine mycelium bearing small, immature vesicles. Another specimen with the 
hyphse and vesicles on the surface is shown in fig. 54. 
Fungus No. 15., (Figs. 57-71.) 
Many of the thick-walled resting : spores of the fungus described above as Fungus 
No. 2, which, though large, are smaller than those of Fungus No. 3, are also occupied 
by intrusive fungi, the mycelium of which had produced numerous spherical spores. 
In so many cases these small spores can be seen to be borne on a mycelium that we 
are justified in assuming that this was always the case. There is nothing to indicate 
that they were in any case spores normally developed in, and belonging to, the large 
thick-walled resting-spore of the original fungus. The enclosed spores may fill the 
cavity more or less completely, and may also be present in the space between the 
contracted inner layer and the outer layer of the wall of the large enclosing resting- 
spore. They, may also burst through the wall of the latter or be formed on its 
exterior. The spores of the intrusive fungus may themselves be occupied by an 
intrusive mycelium bearing still smaller spores. 
While the contained spores thus differ in size, it does not appear possible or 
desirable to carry out a distinction of form-types on this ground, though it is 
probable that we are concerned with a number of forms. An adequate idea of the 
structures in question and the relative positions they occupy will be obtained by 
referring to a number of examples that are represented in figs. 57-71. These 
examples could have been considerably extended without, however, substantially 
adding to our knowledge. 
In fig. 57 three resting-spores of Fungus No. 2 are seen in decayed tissue of 
Rhynia major. There is no intrusive fungus to be seen in the spore on the right, 
but the other two are loosely filled with thin-walled spores. There were only slight 
remains of the hyphse which bore these. The middle specimen is more highly 
magnified in fig. 58. The specimen represented in fig. 59 is similar ; it further 
shows clearly the presence of spores of the intrusive fungus between the layers of 
the wall of the large resting-spore. The thick-walled spore in fig. 60 is large for 
Fungus No. 2, and small for Fungus No. 3. It is filled with spores of the intrusive 
fungus, between which the fine mycelium could be seen, though it is not brought out in 
the figure. The specimen in fig. 61 similarly shows the spores of the intrusive fungus, 
some of which are more highly magnified in fig. 62. In all these cases the thin-walled 
spores approach in size those of Fungus No. 14. This is evident on comparing figs. 49 
and 56 with figs. 60 and 62, the magnification in each case being 250 diameters. 
