CHAPTER VII.—PHENOMENA OF VEGETATION.—LICHENS, 405 
them which have a very thick thallus, as Haematomına ventosum, Mass, and Lecanora 
Villarsii, are distinguished by a very thick medullary layer, sometimes forming according 
to Tulasne 38 of the whole thickness of the thallus. The granulation or areolation 
is produced by the course of growth which has already been described. This is the 
case at least in a number of species examined by Frank and myself which live in 
bark, Lecidella enteroleuca, Kbr. (Lecidea parasema, Nyl.), Ochrolechia pallescens, 
Lecanora pallida and Pertusaria, and it is probable that a large number of other species 
of similar habit agree with them in this respect. 
In these Lichens (Fig. 174) the margin of the thallus consists simply of several layers 
of hyphae, which radiate in the direction of the surface (A) and run parallel to one 
another in a vertical and radial longitudinal section (3). These together form a broad 
marginal zone often of different thickness in different radial strips, and with the extremi- 
ties of single free hyphae projecting beyond its circumference. The peripheral portion of 
this zone, as Frank has pointed out, always grows through outer layers of the periderm, 
and first emerges from them at some distance from the periphery where it is thicker. 
The margin advances by marginal progressive growth and the surface of the thallus 
therefore enlarges. The algal cells which grow and multiply move forwards in the 
same direction behind the margin, and wzthiz the layers of hyphae. They lie collected 
together into groups, and as the algal cells constantly increase in number the hyphae 
are continually forming fresh branches in and around each group, and becoming more 
and more densely interwoven till their originally radial course is at length obliterated ; 
this happens especially on the side towards the substratum. Thus a coil of closely 
interlacing hyphae inclosing the algal cells is formed at the points indicated, the 
extremities of the filaments nearest the surface running chiefly at right angles to the 
surface and forming a rind which is usually very thin (Fig. 174 B). 
The spots in which these formations take place rise above the surface of the marginal 
zone in the form of arched projections, small warts, which increase rapidly in number 
and size with the increase of the distance from the margin till they touch one another 
and become confluent. The mature thallus appears variously uneven, warted, granular 
or areolated according to the shape of the projections, their elevation and the extent to 
which they become confluent. In some forms with a very thick thallus, as Haematomma 
ventosum, Mass. and Lecanora Villarsii, Ach., the areolation of the surface of the thallus’ 
may be caused according to Schwendener by the rind becoming fissured in consequence 
of excessive dilatation of the medulla; this may of course happen independently of the 
first formation of the areolae as here explained. 
Some crustaceous Lichens, as Lecidea (Rhizocarpon, Kbr.) geographica, L. confer- 
voides and Urceolaria cinerea according to Tulasne’s figures, show an edging round 
their distinctly areolated thallus, formed of radiating branched confervoid strands of 
hyphae firmly united together and adhering closely to the substratum. The areolae of 
the thallus are formed in centrifugal succession on this marginal expansion, at first 
as small scales or warts which gradually increase in size till they are in contact with 
each other on every side. We need not assume that there are other differences between 
this mode of development and that first described, besides the one difference that in the 
present case the marginal zone is divided into the separate strands, while in the other 
it is coherent. The structure of the areolae shows no special peculiarities. The 
marginal strands of the last-mentioned forms as well as the marginal zone of those 
which grow in the manner of Lecidella enteroleuca, Kbr., are often under the name 
of hydothallus confounded with rhizoids or root-hairs. The term protothallus as it 
indicates a commencing state is more suitable but is at the same time unnecessary. 
As regards the minuter structure of Lichens, the hyphae of most of the heteromerous 
‘species are comparatively slender. The hyphae of the medulla are usually slenderer 
than those of the rind, and their cells may be very long, as in Usnea, sometimes 
reaching according to Schwendener a length of 200 4; but the hyphae may also 
be short-celled, as in Fig. 172, and their structure may even be pseudo-parenchymatous 
