No. 390.] REVIEWS OF RECENT LITERATURE. 541 
Lichenes. From this a few general statements are culled which may 
be of interest to non-lichenologists. The literature of lichens is very 
extensive, that here cited covering three closely printed pages. 
With very few exceptions lichens are of slow growth and long life, 
several decades being required by some alpine sorts to reach maturity 
and fructification. The greater number occur on rocks, but many 
are found on tree trunks, dead wood, or on the earth. Only a few 
sorts grow under water (Verrucaria sp.). It may be assumed that 
nearly every one knows that lichens are symbiotic growths. It is 
now just thirty years since the publication of Schwendener’s mem- 
orable paper, and the matter was fought over and settled in the 
seventies. In different lichens the relation between the fungus and 
the alga is very different. In Physma, Arnoldia, and many other 
genera the algal cells are destroyed by the fungus, haustoria being 
sent through the algal membrane into the plasma. In other lichens, 
such as Micarea, Synalissa, the haustoria bore through the algal 
membrane but do not penetrate into the plasma. As a rule the algal 
cell is entered by only one haustorium. Finally, in many lichens, 
e.g., those with Protococcus gonidia, the relation of the two com- 
ponents is only one of intimate contact, the fungus causing no vis- 
ible change either in the membrane or in the contents of the algal 
cells. The manner of union of fungus and alga is very constant in 
any given species of lichen, and it is usually the fungus which deter- 
mines the form of the lichen. According to external form, lichens 
may be classified as bushy, leafy, or crustaceous. Formerly much 
use was made of these distinctions for purposes of classification, but 
this system has now been abandoned, as separating closely related 
forms. Classification is now based largely on the ascospores. For 
anchorage and food absorption lichens are united to the substratum 
to very different degrees. The most intimate union is that of crusta- 
ceous, calcivorous species — Verrucaria, Staurothele, Thelidium, etc. 
The hyphz of some of these species penetrate into the stony sub- 
stratum in all directions, to a depth of ten to twenty millimeters or 
more. On the other hand, the Collemas, etc., only rest their gelat- 
inous masses on the substratum without producing any visible 
change in it. Between these extremes are various intermediates. 
Even the hardest rocks are eroded by lichens. Lecanora polytropa 
offers a striking example of this, the hard gneiss rock to which it 
adheres being eroded into cavities exactly the size and shape of the 
lichen crusts which occupy them. In most lichens the algal elements 
are confined to a particular zone, but in some sorts the alge are 
