140 
IMycologia 
he thinks that the relation of these lichens to their algal hosts is a 
very loose one. In the fruticose Roccellas, the Trcntepohlia cells 
are rent asunder and attacked by haustoria as are the usual pal- 
melloid host cells in most higher lichens, but not so among the 
lower lichen parasites. It will be noted that Lindau does not 
agree with Frank regarding attachment of the hypophloeodal 
lichens to the algal host cells. 
Frank found Lecanora pallida Schreb. to be of hypophloeodal 
origin, and to become epiphloeodal later. The unicellular algal 
host cells are always present over the central, thicker portions of 
the thalli. There is a marginal zone of hyphae, from which algal 
cells are absent. The algae occur singly and much scattered just 
inside of this zone, though tetrads may be recognized occasionally 
where the lichen hyphae have not pushed between the cells. 
These algal cells are, in part, pushed forward by the growing 
lichen hyphae and form new tetrads ; the process is continued, and 
the host cells thus follow the growth of the lichen thallus laterally. 
Some of the algae are colorless, but are alive and divide actively. 
These colorless cells occur among normal green ones, but are 
absent from some thalli. Continued growth of the lichen and 
its algal host causes a breaking down of the periderm above, and 
lichen and host become epiphloeodal, except the marginal zone of 
the lichen, which remains hypophloeodal. After becoming epiph- 
loeodal, the thallus acquires a pseudocortex of entangled hyphae, 
and the algal cells come to lie in groups below this cortex. Frank 
always found one or more cracks in the periderm of the area 
occupied by the young thalli. Through these he supposed that 
both the lichen and the algal host enter. He always found the 
algal cells in the cracks in the periderm. These are present and 
are attacked by the hyphae as they enter the cracks. 
We may now pass to a consideration of some ideas regarding 
the physiological relations of the lichen to its algal host. De 
Bary (i6) thought the lichen should be regarded the host and the 
alga a guest, treated with such great consideration that the relation 
should be regarded mutually beneficial. Reinke (65. 104 and 105) 
holds practically the same view and uses to express it the term 
“consortium,” proposed by Grisebach (65). Schneider (109') also 
uses the term “ mutualism ” to express the relation of the lichen to 
