The remaining flowerless plants, having no duct system in their make-up, are, as we know, called _non-vascular cryptogams_. This is a general term for a very large group of plants, some quite obvious and well known like a mushroom, for instance; others so small or of such uncertain structure that they are not even well known by experts. This great mass of plant life, more numerous than all the other kinds of plants combined, contains many different forms, some of which are of gigantic size. A single plant of a certain Pacific Coast seaweed regularly exceeds in length the height of the tallest [Illustration: FIGS. 64-67.--TYPES OF FLOWERLESS PLANTS Fig. 64. A moss plant. Fig. 65. A mushroom, a common type of the fungi, which include also puffballs, molds, and many disease-causing microscopic organisms. Fig. 66. A common seaweed, a representative of the algæ, which include the green scum on the top of ponds, and the kelp from which fertilizer is now being made. Fig. 67. A lichen, a common cryptogamous plant on logs and rocks. Our native kinds are usually grayish-green in color.] known trees. And yet other inhabitants of the water, certain kinds that float freely, are microscopic in size. The latter occur in such enormous numbers that their tiny decomposed skeletons after dropping to the bottom of the sea form the diatomaceous earth, so much used in polishing machinery. The commercial product now comes from deposits of these skeletons laid down in past ages, which, due to changes in the land and water surfaces of the earth, are now found in Virginia, Nevada, California, and in Bohemia. All these must have been in the bed of waters long since gone, which teemed with these microscopic organisms. To-day there are over ten thousand different kinds known, yet so small are they that their dimensions are measured in thousandths of an inch! Somewhat lower in the scale of life--and by this we mean simpler in structure--than the ferns are the _mosses_. (Figure 64.) There are thousands of different kinds, but everyone is familiar with the collective growth of the commoner sorts which makes the velvety mossy carpet in our woods. The individual plants are small, but in many kinds sufficiently large to be seen without a microscope. Most important of all, practically every one of them has the ordinary green color of the better known plants, and as we shall see in the section devoted to “Leaves as Factories for the Making of Food,” that stamps them at once as plants, if other things did not.