Thallus growth forms

Lichens are customarily divided into three growth forms, although this taxonomy is one of convenience, and is not ultimately founded on systematic relationships. Crastose lichens form a thallus that is closely ap-pressed to the surface upon which they are growing. Fo-liose lichens are only joined to their substrate by a portion of their thallus, and they are somewhat leaf-like in appearance. Fraticose lichens rise above their substrate, and are much branched and bushy in appearance. 

The habit of a lichen is due not only to the overall growth form, but is often the result of special anatomical characteristics. For example, the shape of the thallus surface depends on the anatomy of the cortex. External appearance and internal structure are interdependent. 

The thallus of foliose lichens is formed by flattened lobes, which are heteromerous and dorsoventral in structure. Two principal types, the laci-niate and the umbilicate growth form, can be distinguished. Laciniate thalli adhere more or less firmly to the support on which they grow. Either the whole lower surface is in contact with the substrate or the margin of the lobes becomes free and bends upwards. The thalli are usually attached by rhizines or rhizoidal hyphae. The umbilicate lichens are platelike and attached by a central discoid holdfast called the umbilicus (Fig. 47). 

The constitution of the cell walls of the algae and the fungi determines to a certain degree the growth form of a thallus. 

The division into genera in all previously proposed systems is quite artificial. One can find transitional stages in spore septation, in the presence or absence of involucral formation, and in growth form of the thallus. 

The structure and development of the tissues depends on the form of the cells and on the particular type of contact between them. This is achieved either by the mutual adherence of the cell walls, by the formation of anastomoses, or by the gelatinization of the cell walls. Other important factors are the direction of growth and the orientation of the hyphae to the surface of the thallus and to each other (Figs. 5-8). 

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