=The last big family of plants is that containing the simplest plants of all.= They are often very small and apparently unimportant, sometimes so small that we cannot study them at all without magnifying them very much with the microscope. In other cases they are quite large and easy to see; for example, the big red and brown seaweeds, and the many toadstools in the autumn woods. Sometimes they may even be very huge indeed, as are some of the seaweeds which grow in tropical seas. All the same, though we examine one which is as big as can be, it is really more simple in its detail than the mosses. In very ~many of the algæ and fungi, the whole plant body consists only of one single cell~. When this is the case, the plant lives floating or swimming about in water, or in very damp places. In rain-water which has stood for a long time you may find numbers of these tiny algæ. If you put some of the water in a glass tube and hold it against the light you may just see them, with a magnifying-glass, as specks of green, often swimming actively about. The fine green “scum” which floats on many ponds and slow-moving streams consists of masses of these simple plants, in this case generally of forms in which the single cells keep attached together in long rows or chains, forming hair-like plants. Colourless plants of this kind are the fungi, which are often built on the same plan as the hair-like green algæ, only they do no food-building work for themselves, but live as parasites on other things. This is the case in many moulds and the plants which form potato-disease, and, in fact, the greatest number of plant-diseases are caused by such simple parasites. All these plants are very small and simple, and as you can see at once, are not at all to be compared even with the mosses, but there are others which seem to be more complicated, as are the big seaweeds and the toadstools. Let us see how it is they are put in the same family as the simplest plants of all. You can see, even with your magnifying-glass, that they have no special “water-pipes” in what you may call their “stem,” (for want of a better name), but that their whole body is built up of numbers of soft cells all very much alike, which twine in and out, and build a kind of soft weft; they have no really marked out stem and leaves. Look at a toadstool, for example, there is just a stalk and a cap spreading out above ground, while under the ground there are many twining thread-like strands (~see~ fig. 133). [Illustration: Fig. 133. A Toadstool, showing the “cap” and “stalk.” Under the cap are the radiating gills, on which the spores are formed. Thread-like strands under the soil.]