CHAPTER VII. PHENOMENA OF VEGETATION. LICHENS. 405 



them which have a very thick thallus, as Haematomma ventosum, Mass, and Lecanora 

 Villarsii, are distinguished by a very thick medullary layer, sometimes forming according 

 to Tulasne H 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 (B). 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 within 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 hypothallus 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 p ; but the hyphae may also 

 be short-celled, as in Fig. 172, and their structure may even be pseudo-parenchymatous 



