150 
Mycologia 
attached to the substratum, being fastened usually at one point 
only, and are, of all lichens, least dependent upon the substratum 
and most dependent upon the algal host for their nourishment. 
In general, as the proportion of bulk of the lichen above the sub- 
stratum increases, the penetration into the periderm decreases, 
and the lichen becomes more and more dependent upon its algal 
host. In other words, the lower the lichen, the more dependent 
upon the substratum and the less dependent upon an algal host; 
and the higher the lichen in the scale of development, the less the 
dependence upon the substratum and the greater that upon the 
algal host. Since the proportion of contact with the substratum 
decreases as we ascend the scale of lichen development, the higher 
lichens can hardly carry large amounts of nourishment to the algae 
within them. The evolution of lichens has involved a gradual 
change in food supply, and these plants have become more and 
more dependent upon algal hosts. 
The substratic relation of lichens growing on decorticate wood 
are much like those of epiphloeodal forms on bark, whether the 
lichen be crustose, foliose, or fruticose. Lindau (8i) investigated 
Biatorina synthea (Ach.) Mass, on decorticate pine wood. The 
thallus and the algal host lie among the tracheids, which become 
corroded and sloughed off, while the algal cells lie in the cavities 
thus formed. The lichen hyphae penetrate between the tracheids 
and push them apart. Then the alga enters, and one finds in 
section a mass of tracheid fragments, lichen hyphae and algal 
cells, all of which are sloughed off above as the lichen and the alga 
penetrate deeper into the wood. The lichen hyphae pass into the 
tracheids and medullary ray cells, through broken down bordered 
pits and thus bore deeper and deeper into the wood. 
Psora ostreata Hoffm. sends its rhizoids through holes into the 
rotten pine wood on which it grows, and the hyphae enter the 
tracheids through broken down bordered pits. They pass from 
tracheid to tracheid, and penetrate into medullary-ray cells 
through similar pits, thence deeper into the wood. Both tracheids 
and ray cells are often filled with rhizoids. Cladonias on top of 
stumps of coniferous trees easily send their rhizoids to unusual 
depths, entering the cut ends of tracheids, thence passing from 
