This is a curious little plant with a very slender creeping root-stock. From this are sent down small tufts of roots into the damp soil in which the plant grows. From the upper part of the stem arise the leaves, which are bristle-shaped, two or three inches in length, and of an intensely bright green colour. It is interesting to note that in their young state these leaves uncoil very much in the same manner to be noticed in the Ferns. The spore cases are situated at the point where the leaf arises from the root-stock. Externally these are densely covered with brown hairs, and they are just about the size of a small pea. The sporangia are four-celled, and when the time for the dispersal of the spores arrive, the cases split open. The spores are of two kinds, and both sorts are present in the same sporangia. The large spores are confined to the lower portion of the case, the small ones to the upper part. The Pillwort is common in some parts of England, though less so in Scotland and Ireland. It always grows in very damp situations, though it is rarely found submerged, save when this happens as the result of flooding. The Pillwort so often grows mixed up with grass, which in some ways it resembles, that the plant is frequently overlooked. [Illustration: Azolla caroliniana.] _Azolla caroliniana._ Here the generic name is derived from two Greek words—_azo_, “to dry,” and _ollo_, “to kill”—an allusion to the fact that dryness is fatal to the well-being of the plant. The specific name has reference to the fact that the plant is very abundant in Carolina, though as a matter of fact it occurs in other parts of the world. It should be pointed out that the Azolla is not a native of the United Kingdom, but it has become well established in some parts of the South of England. During the summer time the Azolla, which is a floating plant, increases with great rapidity, sometimes completely covering the surface of a lake. At first the fronds, which are exquisitely beautiful, are of a silver-green colour; towards the autumn they assume a bright crimson tint. Underneath the fronds are produced a large quantity of roots which hang down into the water. Two kinds of spores are produced. The microspores are packed away in cases which are provided with curious barbed contrivances. The megaspores bear hooks, and in this way the two processes become attached. The Azolla is a pretty plant to grow in an aquarium, though where it has been able to make itself at home it will sometimes increase so vigorously that all other vegetation is killed. CHAPTER XII THE HORSETAILS