Growth Forms of the Thallus

Crustose lichens never possess a lower cortex. They are attached to soil, rock, or tree bark by the hyphae of the medulla and the contact is so intimate that they are practically inseparable from the substrate. A patch of crusta-ceous lichen may belong to one species and yet be composed of many individuals which have fused together. Simple crustose lichens are homoio-merous. They lack a cortex and are therefore granular in structure. The mycelium spreads over the substrate in a thin filamentous mat enclosing the algae. 

Intermediate Forms between Crustose and Foliose Lichens 

In some crustose lichens elongated, small lobes replace the areoles. These lobes can be fastened to the substrate by the entire lower surface or the margin of the thallus can be free and ascending. Different combinations of these characteristics lead to a variety of described growth types. 

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). 

Intermediate Forms between Foliose and Fruticose Lichens 

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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. 

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