The Thallogens or Thallogenæ include algæ, lichens, and fungi, which are the lowest of the plants, and all very much alike. The algæ are termed “protophytes,” and consist of living cells propagating by subdivision, or union. The thallogens have therefore no distinct axes, leaves, or stomata.
The algæ are thus simply cellular plants found in salt or fresh water, hot and cold. They sometimes appear as “slime.” Some contain silicia, and are termed Diatomaceæ, and these propagate by subdivision, and when they die their shelly covering remains, and we find the shells or cases in all earthy formations. These diatomaceæ have been raised from the beds of oceans, and Atlantic soundings have revealed their presence,—as mud, when examined, proves to be these remains of vegetable shells. Thus the infinitely little in the animal and vegetable worlds meet at the bottom of the sea, as well as on dry land.
There are marine and fresh-water algæ—the former familiar to us as seaweeds which possess air-bladders that children love to explode, and which assist the algæ to float. They attach themselves to rocks, generally at the base; the lovely colours of seaweeds are well known. They will be recognized under the name of “tangle” (fucus), which, when burned, gives kelp and barilla, which is full of iodide and sodium. The Sargasso Sea is composed of miles of algæ which live in the open ocean. The Carrageen or Irish moss is very nutritious and useful in consumptive cases. Indeed, all algæ, if not absolutely useful, are certainly not deleterious. The “bladder-wrack” was formerly useful for the production of soda.
Fig. 820.—Bladder wrack.
Fig. 821.—Lichen.
“The life-history of one of these uni-cellular plants in its most simple form, can scarcely be better exemplified than in the Palmogeœa macrococca, one of those humble forms of vegetation which spreads itself as a green slime over damp stones, walls, etc. When this slime is examined with a microscope, it is found to consist of a multitude of green cells, each surrounded by a gelatinous envelope; the cell which does not seem to have any distinct membranous wall is filled with granular particles of a green colour, and a ‘nucleus’ may sometimes be distinguished through the midst of these. When treated with tincture of iodine, however, the green contents of the cell are turned to a brownish hue, and a dark-brown nucleus is distinctly shown. Other cells are seen, which are considerably elongated, some of them beginning to present a sort of hour-glass contraction across the middle; in these is commencing that curious multiplication by duplicative subdivision which is the mode in which increase nearly always takes place throughout the vegetable kingdom.”[37]
Lichens are numerous, and may be found upon the bark of trees in dry forms of grey and yellow growth, and on walls and old stones in our graveyards. On the hills we find them growing upon the granite, and it would appear that they prefer stone to any other holding ground. The Arctic lichens form the principal food of the useful reindeer, and “Iceland moss” is represented as wholesome for man. Lichen is derived from the Greek term for “wart.”
The Fungi are very important, and with them we will close our summary. They include the favourite mushrooms and poisonous toad-stools, with many other “fungous growths,” from the “mould” on the jam pot to the mushroom.