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

BOOK 2

together end to end. Spirogyra is an example well known to microscopists ; it forms slimy, felt-like masses on the surface of ponds, and its cells are marked with characteristic green spirals. And from these simple filamentous forms there is an equally gradual transition to the largest and most elaborate alge, the seaweeds of our shores.

A few of the seaweeds are green in colour —the sea-lettuce, ulva, is an example—but in most of them other pigments are present in addition to chlorophyll, so that the colour of the latter is masked and obscured. Their classification is based to a large extent on these extra pigments ; thus we speak of the blue-green alge, the red alee, and the brown alge, as distinct from the green alge already mentioned.

The blue-green alge are simple in structure, some single-celled, some filamentous, some aggregated into gelatinous globules, and do not reach the size or elaboration found in the red and brown alge. ‘They are widely distributed in freshand salt-water. They multiply and swarm like other microscopic organisms, and in fresh-water reservoirs their rapid development, followed by death and decay, may cause serious trouble by giving rise to unpleasant tastes and odours. The plants which give the Red Sea its name belong to this group. The brown alge, represented by the very common bladder-wrack, with its gas-bladder floats, that is exposed in masses on our shores at low tide, and by the ribbon-like fronds of Laminaria that grow by the extreme low-tide mark, include the largest members of the group. The Laminarias that growon the Pacific Coast of America may be over a hundred feet in length, those from the Antarctic may be twice as large as this. And there are other kinds in the Antarctic which grow like drooping submarine trees, several yards in height and having a main trunk as thick as a man’s thigh. The brown alge are among the few members of the group that are of use to man; some species are eaten in China and Japan, and_ before the development of modern industrial chemistry they were used as sources of iodine and potash—indeed, iodine is still extracted from them in Ireland—and they are often used as fertilizers. ‘The red algee (whose colour may be red, violet, or purple), like the brown, are nearly all marine ; they are a varied group, but do not attain the great size or the economic usefulness of the brown.

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THE SCIENCE OF LIFE

CHAPTER 6

§ 4 Moulds, Toadstools, and Yeasts

It is not true that all plants possess chlorophyll. Most plants possess it ; but there are exceptions, organisms with an anatomical structure like that of the humbler green plants, but not themselves green, living on the organic substances that their green neighbours have built up.

Everywhere on the face of the world, except in lifeless regions and in a very few places where civilized man has built, there are the corpses of living things. In a wood, for example, there are masses of dead leaves, which, with other organic matter, make a rich rotting layer on the ground. And wherever the corpses are found we find also a host of creatures that feed on their luscious decaying tissues, animals and plants that specialize, so to speak, in well-hung food. There are beetles and millipedes and worms and grubs of many sorts and there are bacteria, all fattening on the dead material ; but the guests that concern us now at this banquet are the delicate white or bluish felts that are known as moulds.

An attractive provender for moulds is stale, moist bread. Ifa bit of bread be left about somewhere exposed freely to the air, there will soon settle on it a number of the invisibly small, dust-like spores of mouldfungi. Finding themselves well situated, the spores will hatch, and each will give rise to a very thin, white thread. The threads grow and branch and tangle together into a soft feltwork, on the surface of the bread and penetrating into it. They are not subdivided into cells; their protoplasm forms a continuous cylinder, with numerous nuclei dotted about. Seen through a microscope, this protoplasm is revealed to be in continual restless movement, streaming about inside the walls of the delicate threads. This tangle of white threads is a simple kind of fungus. It has no chlorophyll. It needs organic food, which it gets from the bread in a curious way—by exuding a digestive juice and sucking in a nutritious, digested soup in return.

In fact, it digests like an animal, and the whole world is its stomach, into which it pours enzymes and from which it absorbs the products of their action.

From time to time a branch of one of the threads rises up into the air, and it grows at its free extremity a little globule—black, in the commonest bread-mould. ‘This globule is a spore-case, and in it hundreds of tiny spores are developed, to ripen and be