An interesting occurrence is the petrifaction of masses of these plants together, which indicate the existence of an ancient pool in which they must have grown in abundance at one time. A case has been described where masses of _Chara_ are petrified where they seem to have been growing, and in their accumulations had gradually filled up the pond till they had accumulated to a height of 8 feet. The plants, however, have little importance from our present point of view. 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. [Illustration: 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_. [Illustration: Fig. 120.—Fossil Leaf _l_ with Nests of Infesting Fungal Spores _f_ on its lower side]