The science of life : fully illustrated in tone and line and including many diagrams

BOOK 2

8 4 The Flowering Plants

It is by no means true that all plants reproduce themselves by means of flowers and distribute themselves by means of the resulting seeds. These matters are characteristic peculiarities of one division of the vegetable lingdom. It is true that division includes by far the greater number of the plants that now cover the earth with meadows and jungles and forests, and nearly all that are grown for beauty, or for food, or for timber ; nevertheless, there are other plants with other methods, and with these we shall have to acquaint ourselves.

But before we do so we may note briefly that the division with which we have hitherto concerned ourselves, which may be called either the flowering plants (phanerogams) or seed-bearing plants (spermatophytes), is divided into two classes.

The first class includes all the coniferous trees and shrubs—fir, pine, larch, cedar, cypress, juniper, yew, and the rest—and also the peculiar tropical cycads and one or two other curiosities of which we shall learn more hereafter. The conifers bear their pollen or egg-cells on little catkins, and the female catkin grows into a woody cone, each scale of which bears two seeds, but we may note two well-known exceptions to this rule ; in yew and juniper, which are classified as conifers for other and weighty reasons, the fruits are fleshy and distributed by birds like other berries. The definitive characteristic of this class of seed-plants is that the seeds are not enclosed in special ovaries but sit exposed at the bases of the scales ; for this reason they are called gymnosperms, which means “‘ naked-seeded.”’

In the second class the seeds are housed in ovaries, and the members of the group are therefore called angiosperms, which means “with seeds in containers.” The class includes nearly all of the herbs and trees with which the reader is familiar. They belong to the contemporary world. There are no phanerogams in the most ancient rocks ; gymnosperms appear in association with reptiles ; and the angiosperms, flower and grass and leaf-shedding tree, only became dominant as the mammal and bird became prevalent in the world.

85 Ferns and Mosses

We turn now to plants that do not possess flowers, and do not distribute themselves

162

THE SCIENCE OF LIFE

CHAPTER 5

by means of those elaborate and sexuallyproduced structures we call seeds.

The reader will be familiar with the appearance of one or two kinds of fern, but he or she has probably only met the comparatively unassuming members of the group that inhabit temperate countries. For the ferns attain their fullest luxuriance in tropical climates. In Central America the common bracken fern towers to double the height of a man, and there are species peculiar to the tropics that grow like trees, with erect cylindrical stems sixty feet or more in height, bearing crowns of spreading fronds. The group is large and varied ; it includes over three thousand species, ranging in size from these tree-ferns to the minute filmy ferns, hardly larger than mosses.

Instead of bearing flowers and seeds, ferns reproduce themselves by means of spores. A spore is a tiny particle of living substance, enclosed in most instances in a more or less resistant shell, and capable of growing directly into a plant without any sort of fertilizing process. It is a sexless production. Fern-spores are produced by proliferation of cells in the pale brown areas that form such conspicuous patternings of circles or stripes on the lower sides of the fronds of most ferns. The spores are very small and light ; indeed, they are single cells, and when they are ripe they escape and are blown about, often for considerable distances, by the wind.

If a spore drops on suitable soil it germinates and begins to grow, but it does not develop into a new fern. It grows into a structure called a prothallus, which, instead of bearing conspicuous erect fronds, is, in most cases, a flat leaf-like plate of green tissue, less than an inch across, and sending a few tiny roots into the soil. This simple object is not a young fern—indeed, it is not a young thing at all, for it is sexually mature; it reproduces and dies without further development. It isa generation initself. Scattered over its under surface there are tiny, reproductive organs of two kinds, male and female. The female organs are shaped like longnecked flasks, and waiting in the belly of each flask is a single egg-cell. The male organs are simply globular masses of tissue that is proliferating and giving rise to the male elements. Now the male elements are not passive, like pollen-cells, but they swim actively like spermatozoa. They have little bodies, shaped like a corkscrew and with a tuft of lashing tails at one end ; at the other they usually carry hollow, _bladder-like structures which may come off and be left behind without in any way disconcerting the