In the majority of lichens the fungal tissue constitutes the bulk of the thallus, as suggested by the simple diagrams near the top of this page. There is a small number of genera in which photobiont tissue dominates. A number of byssoid genera, with cotton-wool-like thalli fall into this category as do some more robust genera such as Collema and Leptogium. In the latter two the photobiont is cyanobacterial in the genus Nostoc and the thalli of Collema and Leptogium could initially be mistaken for Nostoc, especially if the lichens lacked apothecia. The following photos show Nostoc (left) and a fertile Collema (right). Without the white apothecia the Collema thallus is featureless and gelatinous in texture, just like a Nostoc.
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Given that fungal tissue is usually dominant it would be understandable to conclude that, in such cases, it is the fungus that determines whether a thallus will be crustose, foliose or fruticose. However, the case of STICTA AND 'DENDRISCOCAULON' is instructive. It shows one fungus associating with different photobionts (one an alga, the other a cyanobacterium), but without the cyanobacterium confined to cephalodia. Instead there is a dimorphic thallus, with one form of thallus containing the alga and the other form the cyanobacterium, so that thallus form is influenced by the photobiont. Note that in Cladonia and Stereocaulon the different components of the dimorphic thallus have the same photobiont, though you will also find cephalodia in many species of Stereocaulon.
In the majority of lichens the fungal tissue constitutes the bulk of the thallus, as suggested by the simple diagrams near the top of this page. There is a small number of genera in which photobiont tissue dominates. A number of byssoid genera, with cotton-wool-like thalli fall into this category as do some more robust genera such as Collema and Leptogium. In the latter two the photobiont is cyanobacterial in the genus Nostoc and the thalli of Collema and Leptogium could initially be mistaken for Nostoc, especially if the lichens lacked apothecia. The following photos show Nostoc (left) and a fertile Collema (right). Without the white apothecia the Collema thallus is featureless and gelatinous in texture, just like a Nostoc.click to enlarge click to enlargeGiven that fungal tissue is usually dominant it would be understandable to conclude that, in such cases, it is the fungus that determines whether a thallus will be crustose, foliose or fruticose. However, the case of STICTA AND 'DENDRISCOCAULON' is instructive. It shows one fungus associating with different photobionts (one an alga, the other a cyanobacterium), but without the cyanobacterium confined to cephalodia. Instead there is a dimorphic thallus, with one form of thallus containing the alga and the other form the cyanobacterium, so that thallus form is influenced by the photobiont. Note that in Cladonia and Stereocaulon the different components of the dimorphic thallus have the same photobiont, though you will also find cephalodia in many species of Stereocaulon.
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