Fungi.—Of the higher fungi, namely, “toadstools”, we have no true fossils. Some indications of them have been found in amber, but such specimens are so unsatisfactory that they can hardly afford much interest.
Fig. 119.—The Hyphæ of Fungi Parasitic on a Woody Tree
c, Cells of host; h, hyphæ of fungus, with dividing cell walls.
The lower fungi, however, and in particular the microscopic and parasitic forms, occur very frequently, and are found in the Coal Measure fossils. Penetrating the tissues of the higher plants, their hosts, the parasitic cells are often excellently preserved, and we may see their delicate hyphæ wandering from cell to cell as in [fig. 119], while sometimes there are attached swollen cells which seem to be sporangia. From the Palæozoic we get leaves with nests of spores of the fungus which had attacked and spotted them as so many do to leaves to-day (see [fig. 120]). What is specially noticeable about these plants is their similarity to the living forms infesting the higher plants of the present day. Already in the Palæozoic the sharp distinction existed between the highly organized independent higher plants and their simple parasites. The higher plants have changed profoundly since that time, stimulated by ever-changing surroundings, but the parasites living within them are now much as they were then, just sufficiently highly organized to rob and reproduce.
A form of fungus inhabitant which seems to be useful to the higher plant appears also to have existed in Palæozoic times, viz. Mycorhiza. In the roots of many living trees, particularly such as the Beech and its allies, the cells of the outer layers are penetrated by many fungal forms which live in association with the tree and do it some service at the same time as gaining something for themselves. This curious, and as yet incompletely understood physiological relation between the higher plants and the fungi, existed so far back as the Palæozoic period, from which roots have been described whose cells were packed with minute organisms apparently identical with Mycorhiza.
Fig. 120.—Fossil Leaf l with Nests of Infesting Fungal Spores f on its lower side
Algæ.—Green Algæ (pond weeds). Many impressions have been described as algæ from time to time, numbers of which have since been shown to be a variety of other things, sometimes not plants at all. Other impressions may really be those of algæ, but hitherto they have added practically nothing to our knowledge of the group.
Several genera of algæ coat themselves with calcareous matter while they are alive, much in the same way as do the Charas, and of these, as is natural, there are quite a number of fossil remains from Tertiary and Mesozoic rocks. This is still more the case in the group of the Red Algæ (seaweeds), of which the calcareous-coated genera, such as Corallina and others, have many fossil representatives. These plants appear so like corals in many cases that they were long held to be of animal nature. The genus Lithothamnion now grows attached to rocks, and is thickly encrusted with calcareous matter. A good many species of this genus have been described among fossils, particularly from the Tertiary and Cretaceous rocks. As the plant grew in association with animal corals, it is not always very easy to separate it from them.