FINK—-THE LICHENS OF MINNESOTA. 23 
The hyphe in all portions of the thallus, however, may increase in 
length, either by the formation of new cells or by the elongation of 
old ones. In some instances the initial cells are equally active along 
the whole margin and the thallus is not lobed. But in some of the 
higher crustose species and the majority of the foliose species there are 
certain marginal areas where the initial cells are especially active, so 
that the horizontal growth is more active here than elsewhere. This 
uneven growth along the margin gives rise to the lobing so common 
in foliose lichens. Doubtless in the lower crustose lichens, where 
lobing is so uniformly absent, the frequent irregularity in form is due 
partly to irregularities in the surface of the substratum rendering 
growth more difficult at certain points than elsewhere, and in part also 
to loss of portions of epiphlaodal or epilithic thallus by abrasion. In 
all instances of lobing the form of the lobes is determined by the size 
and amount of activity of the areas of initial cells whose division forms 
the lobes. The thallus reaches its full thickness a short distance back 
of the growing margin. There is no further increase owing to the 
fact that, while new cells, both algal and fungal, are formed internally, 
abrasion of the general surface disposes of dead superficial cells to the 
same amount. 
In most fruticose thalli the growth is mainly vertical instead of 
horizontal. There may be a single apical region, consisting of a con- 
tinuous area of initials cells, and in such instances the thallus or the 
podetium will be unbranched. But in the great majority of instances 
there are areas of special activity, one corresponding to each branch 
of the thallus, whether horizontal, vertical, or lying at some inter- 
mediate angle. Here, as in all thalli, the manner of branching and 
the number of branches will depend upon the number and disposition 
of these areas of special activity. 
But in some instances the growth of the thallus is essentially a 
erowth of the algal instead of the fungal symbiont. This is the case 
in our Ephebe pubescens (pl. 24, facing p. 147), in which the alga 
branches dichotomously, has a single apical cell at the end of each 
branch, and determines both the growth and the form of the lichen.* 
In Collema and Synechoblastus, where the algal cells are somewhat 
evenly distributed throughout the thallus, it is not very clear whether 
the algal or the fungal symbiont has more to do with determining the 
growth and form of the thallus. But since the growth is marginal 
and the irregular lobing seems to correspond to areas along the margin 
especially rich in algal filaments, it appears that the form is due mainly 
to the growth of the alge. In the great majority of lichens, however, 
as already stated, the algal cells simply increase in number as the thal- 
lus increases in size. Thus the alge always sustain a physiological 
a That is, according to the ordinary conception; but the real thallus is fungal and 
within the alga. 
