SCHOOL IN THE OPEN.

Star (Christchurch), Issue 18889, 12 October 1929, Page 23 (Supplement)

SCHOOL IN THE OPEN.

STUDIES IN THE GREAT OUT-OF-DOORS.

(By J. J. S. Cornes, 8.A., B.Sc.)

The “ Star ” has arranged with Mr J. J. S. Cornes, 8.A., B.Sc., to write a series of illustrated articles which will give teachers and others a fuller aporeciation of the Great Out-of-Doors. They will deal with various aspects of plant and animal life, as well as with inanimate nature. Questions and material for identification will

be welcomed.

THE VEGETABLE KINGDOM. BIOLOGY XVI. CCCXI. In classifying the green plants—we will disregard the fungi—botanists use the same principles as do zoologists with animals, and in the same belief. Those plants which most closely resemble one another are grouped together, the endeavour being to indicate by the grouping the blood-relation between the plants, and the course of their evolution. 1. Thallophyta—The unicellular green plants and the sea-weeds (algae) are grouped together as thallophytes. Their cells exhibit little, if any, differentiation; and the whole plant is never differentiated into stem (axis) and leaf. 2. Bryophyta—This group includes liverworts and mosses. Liverworts usually have a flattened form, are still thalloid; while the mosses have acquired clearly differentiated stem and iiniiii!iiiiniiiiniiiuiiini!ii!iiii!iiiiniiiiiiinniiii!iii!iiii!iiiiiiifnii!iii!ii!iiiiiiuiiiii!

much elongated cells called tracheids, from .which the protoplasm has disappeared, and which serve as pipes for the flow of the sap. The tracheids are closed at both ends, but as they are tapering, and dovetail in between those above and below them, the sap can diffuse through any part of the wall of each into neighbouring ones. 4. Spermophyta—These are the seedbearing plants. None of the plants in the preceding groups reproduces by means of. seeds. Sexual reproduction in plants is essentially the same as in animals —namely, the fertilisation of . a single female cell, an ovum, by a single male cell, a sperm. In the preceding groups the young plant that results from the development of the ovsperm is not provided with a store of food, nor protected with special coverings, by the parent. In this group, however, the new generation is so provided protected; and we call the sleeping baby plant a “seed.” The additional care taken by these higher plants to secure the safety and welfare of their offspring by covering them with seed-coats, and providing

A, under-surface of a small portion of a shield-fern leaf, showing the brown patches where sp6res are produced; B, section of one of these patches showing the cover protecting the stalked sacs which contain the spores; C, one of the sacs after it has burst open and scattered its spores; E>, the prothallus, the sexual generation, seen from below, showing “roots” and female organs (ovaries) at the base of the “roots,” with male organs (spermaries) on each side of the “roots”; E, an ovary, with ovum in swollen lower part, and the canal down which the sperm swims to fertilise the ovum; F a spermary, each cell in it containing an immature sperm, shown mature and active to the right. (From Watron, cited by Latter.) iiliinmminininiiiiiiiiinnmimiiiiiiiiiiniimlinniiiniiiiiiiiimmiiiiiimniiiniimanuminniiniiiiiiiilimiiiiininiiimiiiiimiiiiniimiiuiiiiiiiiimmimiiuniiiiimu

leaves; but neither have yet those greatly elongated “cells” with thickened walls and devoid of protoplasm, -vhich in higher plants serve as tubes 'or the transport of fluids and dissolvd food-substances.

them with capita! wherewith to start life’s business, is comparable to that bestowed upon the young by the higher animals. While in birds the yolk of the egg is the only food-store for the developing chick, and the eggshell its only protection, in mammals protection is afforded by the mother’s body itself as the unborn embryo rests in her womb (like growing seeds in their ovary) and food is provided by the diffusion of food-substances from the blood of the mother into that of

3. Pteridophyta—Here we group toether the club-mosses (lycopodium, elaginella, etc.) and the ferns. Here ".here is always a differentiation of the olant into definite stem and leaf; and ; n the stems and leaves there are bundles—the veins in fern leaves —of 'nimminnuniiiMiniifliiniiiiniifiuiniiimiimniiiiiiuiiiniiiiiiiiiimiiiuiiiinfm

the embryo (like diffusion of sap into 1 the growing seed). ~ . The spermophytes are divided into two classes according as the seeds are naked (i. e „ protected by seed-coats only) or further shielded by a special leaf, the carpel, wrapped about them to form a seed-box. (a) Gymnosperms—This naked-seed-ed division comprises yew, pines, firs and all other confers. The water-tubes-in the wood of these resemble those of ferns— that is, they are tracheids. (b) Angiosperms—Here the seeds are enclosed in a case (Greek angeion, a box) formed by the wrapping around them of a leaf (carpel) on which u'nated the single cell (ovum) from which each embryo plant developed. The water-tubes in nearly all of these are true “vessels,” not tracheids, formed by the junction end to end of very elongated cells, from which the protoplasm has been withdrawn and whose ends have been absorbed, so that there is an uninterrupted, continuous passage along each tube. The arrangement is not unlike that of a watermain composed of successive lengths of iron pipes. The angiosperms include all the flowering, or seed-setting, plants other Hhan gymnosperms. Alternation of Generations. We have thus far made no mention of a phenomenon that occurs in the life-cycle of all green plants except the lowly thallophytes, and that illustrates in another way the increase in care bestowed upon the offspring by the plants most specialised in structure, the spermophytes. All green plants, then, except thallophytes, in the course of their life-cycle reproduce in two different ways: (i.) Asexually, by means of single cells (spores) which develop without being fertilised, for they are sexless, neutral; (ii.) sexually, by means if ova fertilised by spermatozoids, or nuceli reprsenting spermatozoids. The plant resulting from the development of the spore is quite different from that produced by the fertilised ovum; and it is on this spore-produced plant that the sexual organs appear. There are thus two generations occur-' ring alternately—the spore-produced plant (A) forms in its body male and female organs, whose fertilised ovum grows into the ovum-produced plant (B) which in turn produces in its body sexless spores—each spore, if fortunate, developing into a generation like (A) with which we started.

Life-Cycle of Fern. As an example of such “alternation of generations” let us take the lifecycle / of a fern. On the backs of the leaves of a fern are numerous brown patches which contain masses of spores. When ripe, the spores are scattered, and such of them as alight in fairly damp and shaded spots germinate and grow into small, flat, green, heart-shaped plants (A) called “prothalli.” These are very like liverworts in general appearance, and can generally be found on the damp brickwork under the stages in a greenhouse where ferns are kept. Each prothallus is quite self-support-ing. It thrusts thin, colourless “roots” into the soil to procure water and dissolved mineral salts, while, in virtue of its chlorophyll, it takes in carbon dioxide from the air after the manner of all green plants. On its under side it develops male and female organs; active spermatozoids swim, by means of their flagella, from male organs to female organs, which they then enter, and their nuceli fuse with the nuceli of the ova within—one spermatozoid nucleus with one ovum nucleus.

The ovum thus fertilised develops; at first it depends for nourishment on its parent, the prothallus; but soon it establishes for itself root and stem and leaves and grows independent, becoming the fern plant (B) that we all knuw; and the cycle begins again with spores on the back of its leaves.

In our next article we shall try to show that even in flowering plants there is the same alternation of generations, though much disguised; that the following tree, like the tree-fern, is really the spore-bearing generation (B), while its male pollen-grains and female ovules are the sexual generation (A), being diminutive prothalli produced by spores germinating on (B) itself instead of falling to the ground. (To be continued next Saturday.) Answers to Correspondents. To “Student”—The flower you send is “Star of Bethlehem,” ornithogalum umbellatum, obviously a lily, and closely related to the taller “Onion Flower,” O. mutans. To Sister Canice, Ross.—Have replied by letter. To A.F., Pelorus.—Have replied by letter.