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have distinct stem and leaves, like the mosses, though the leaves are of a more delicate texture, and lack the mid-rib which many of the mosses possess. The germination of the spores is similar to that of mosses.

The Liverworts (Marchantiacea) have no distinct stem or leaves, but consist of a flat, horizontal cellular frond, attached by tiny rootlets from the under surface. There are but nine British species, the most plentiful of which is the Common Liverwort (Marchantia polymorpha), which will serve to illustrate the order. It is very common in all moist

an

FIG. 110.

FIG. 109.

situations, its dull green fronds covering patches of earth. The organs of reproduction are borne upon ́stalks. The antheridia and archegonia are produced on separate plants. The antheridia receptacle is shown in fig. 109. It is slightly concave on the upper surface, and studded with the openings of the antheridia, which are embedded in its substance (fig. 110, section). They contain the antherozoids, which, on being set free, enter and fertilise the archegonia, which give rise to the sporangia shown in fig. III. The spore cases are arranged on the under surface

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of the receptacle, and contain spores mixed with elaters, as in the Scale-mosses. The fronds also

FIG. 111.

bear upon their upper surface little cups. containing spore-like bodies, which are really buds, or gemma, and capable of developing into perfect plants. The purpose of these buds is evidently to perpetuate the plant when it is growing under conditions unfavourable to the production of spores.

Formerly this species had a great reputation for its supposed medicinal virtues. Old Culpepper says of it: "It is under the dominion of Jupiter, and under the sign Cancer. It is a singular good herb for all the diseases of the liver, both to cool and cleanse it, and helpeth the inflammations in any part, and the yellow jaundice likewise Being bruised and boiled in small beer, and drunk, it cooleth the heat of the liver and kidneys."

The remaining order of this group of plants is known as the Ricciacea or Crystalworts. It is a very.

FIG. 112.

small order, containing only six British species. They are somewhat similar in structure to the Liverworts, but simpler. The spore cases are not stalked, but borne upon the surface of the frond, or embedded in it, as in

fig. 112. The spores are not mixed with elaters in this group, neither do the spore cases split into valves.

The Lichens are more nearly allied to the Fungi,

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though in form many of them resemble some of the Liverworts. Yet though allied to the Fungi, there is no close relation to them. Fungi derive their nourishment from the substances upon which they grow. Not so Lichens. Their food is absorbed from the atmosphere, and it may be taken as a general rule that where Lichens grow the atmosphere is pure, for any impurity kills them. They have been observed to disappear entirely from districts where they formerly occurred in great abundance; and such disappearance has been entirely due to the pollution of their atmosphere by the growth of a manufacturing town, the establishment of a colliery, or other smoke-producing industry. Mr. W. Johnson, in a recent volume of "Science Gossip," gives an instance of the disappearance of a species from the woods of Gibside, Durham. "In Winch's Flora of Northumberland,' published in the Trans. Nat. Hist. Society of Northumberland and Durham, 1832, mention is made of a number of lichens growing in woods at Gibside, Durham. Amongst the plants enumerated is Evernia prunastri, said to be in fructification in Gibside Woods. As I have never had the pleasure of gathering this species in fruit in any part of North Durham, or the west and south of Northumberland (which I have more or less searched) I went out to Gibside in the spring to see if I could. find the above lichen. Gibside is some seven miles from Newcastle to the south-west. The hall is beau

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tifully placed on the Derwent. The surrounding woods run back on to Whickham Fell. On the latter I found one or two forms of Callema, and what

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so a tow of the commoner forms of Lecanora

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@side Woods, barring the atmoarourable enough for the growth of frucfoliaceous lichens, but for any of these I searched in vain. Not a trace of the series Kim could I find. The trees were as barren

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Ramalina, and Evernia as if they had

never known them, and I might say of almost every

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few barren patches of the thallus of some Calicium; and I noticed a few forms of Lecanora and Lecidea

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the river side.

The lichens which flourished here in the fine condition spoken of by Winch have perished, and this evidently from the pollution of the atmosphere by the smoke and fumes from the Tyneside, and the collieries of the surrounding district. Though these are a considerable distance from Gibside, yet the deleterious elements travel on the wind, for the trees have that dusky coating on their trunks and branches which is peculiar to trees bordering a town, and which is fatal to lichen-growth." "'*

The development of these plants is exceedingly slow, and they take years to come to maturity, owing greatly to the fact that their growth is suspended during dry weather. There is little necessity to describe them, as all are acquainted with their grey or yellow incrustations on the rocks of the mountain side, the trunks of trees, and even upon brick walls. But beneath this sober coat of grey they possess a *Science Gossip, 1879, p. 217.

layer of green cells, which are capable, when set free, of giving rise to new individuals. These green cells are known as gonidia (fig. 113, G), and concerning them a humorous theory was promulgated a few years ago, but met with the ridicule it deserved. Professor Schwendener, the author of this theory, believed that these gonidia were neither more nor less than alga-little plants allied to the protococcus described in Chapter I.—which had been captured by a fungus, and made to provide nutriment for their captor. But we will give the learned Professor's own words :—

"As the result of my researches, all these growths are not simple plants-not individuals in the usual sense of the term; they are rather colonies, which consist of hundreds and thousands of individuals, of which, however, only one acts as master, while the others in perpetual captivity provide nourishment for themselves and their master. This master is a fungus of the order Ascomycetes, a parasite which is accustomed to live upon the work of others; its slaves are green alga, which it has sought out, or indeed caught hold of, and forced into its service. It surrounds them, as a spider does its prey, with a fibrous net of narrow meshes, which is gradually converted into an impenetrable covering. While, however, the spider sucks its prey, and leaves it lying dead, the fungus, incites the algæ taken in its net to more rapid activity; nay, to more vigorous increase."

It is unnecessary here to state the facts which prevent the acceptance of this theory: we, in fact, should not have thought it necessary to allude to the subject

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