- Lichens are composed of a large fungus with many single-celled photosynthetic microbes living within.
- In some varieties of lichens, the photosynthetic microbes are green algae, in others, they are cyanobacteria.
-The fungal part of the lichens forms the body structure and most of the mass of the organism.
- The fungal structure is very tough and hardy - lichens can live happily in some of the harshest conditions on earth: desert rocks, arctic tundra, volcanic rocks, even toxic slag heaps!
- They are often seen growing right on top of a dry rock! No need for soil at all!
- Unlike most other fungi, fungal lichens do not live within their food source. Rather, the "food" is made for the fungus by the photosynthetic green algae. For this reason, the shape, structure, and appearance of lichens do not resemble any other fungal species, and actually more closely resembles simple plants. Further, the lichens fungal organisms are completely dependent upon the green algae that live within.
- The algae part of lichens benefits from the tough, protective fungal structure and is provided with water, minerals and ions by the fungus. Thus, the algal species that are found within lichens have also evolved to be dependent on the fungal relationships and they cannot survive separately. Nevertheless, the cells, genes, and reproduction of the algae and the fungus are entirely separate - they remain two distinct species.
(1) Parenchyma - The most common type of ground tissue, parenchyma cells form the cortex and pith of stems and roots, the cortex of roots, the mesophyll of leaves, the pulp of fruits, and the endosperm of seeds. Parenchyma can function in nutrient storage, metabolism, water balance, and wound healing.
(2) Collenchyma - form long thick layers for strength and support of growing leaves and stems, especially in plants that need to stand upright.
(3 ) Sclerenchyma - provide the support and strength in areas of plants that are no longer growing. When mature, these cells are actually dead and have very thick secondary cell walls. They are most commonly found in wood.