Byron David Halsted

“ The fact that the largest tree and the smallest herb are alike made up of a greater or less number of these plant-members, as they are termed, leads naturally to the thought that any mass of plant-tissue having a root, a stem, and a leaf, may be a plant individual, and that, when a number of these members are intimately associated together, a community of plant individuals is formed. ”

“ THE sexual generation of a plant is that stage in its life-history which bears the male and female organs, while the asexual generations are those having no sexuality manifest. The two kinds of generations frequently follow each other in alternate order, when there is what is known as an alternation of generations.
Growing plants continue to increase in size in a well-defined manner for a time, and then a single cell, or a small group of cells, begins on a new line of development. This new growth finally becomes detached from the parent-plant. ”

“ The adult fluke in the liver of the sheep produces enormous numbers of eggs, which are distributed with the droppings of the sheep. If these eggs have moisture and a suitable degree of warmth, they continue to live, and in each is formed an embryo. The embryo leaves the egg and swims in search of the particular snail (Limnæus truncatulus) within which its future life and growth take place. The embryo bores into the snail, and then grows into the form which is called a sporocyst. The sporocyst gives rise to the second generation. ”

“ The seat of vitality is protoplasm, and wherever there is enough of this vitalized substance to grow and reproduce its kind, there we have an individual—a unit of life. It may be concentrated in a single cell, or distributed through many cells, the number and distribution being determined by the amount of dependence of the growing cells upon each other. The greater the division of labor, the higher the form of life, and the more difficult to recognize the individual ”

“ The ovule, as the female cell with its immediate surrounding tissue is termed, becomes inclosed with one or more coats of varying thickness, and, when the whole structure has reached maturity and is ready to separate from the parent-plant, we have the familiar body known as a seed. The seed is an independent plant-structure designed to develop into a mature herb, shrub, or tree, when the conditions are favorable for its germination and growth. ”

“ Among gymnosperms it is not difficult to see that it consists of the pollen-grain and the embryo sac with its endosperm. The male prothallus is reduced and rudimentary, and one cell of the pollen-grain, representing the antheridium, produces a tube instead of antherozoids. The endosperm is the female prothallus, and in it the germ-cell develops. The asexual generation is the plant that grows from the fertilized germ. In the angiosperms the sexual generation is reduced to its simplest form, namely, a single cell for the male part, and one or a few cells for the female. ”

“ In the operation of grafting, a similar portion of a plant community of one variety is given a fitting place for growth in another, and by its growth and multiplication of phytons a new colony is established. With this view a tree or shrub may form an individual part of a landscape, but not in the same sense that one may speak of a cow or a horse. ”

“ Not only does the race of the liver-fluke multiply and increase abundantly in the sheep by producing myriads of eggs, but there is a further and great increase of the forms within the snail. If only the greatest degree of ordinary increase were reached, a single fluke might give rise to more than a hundred million descendants in the next generation of liver-flukes proper inhabiting the sheep. ”

“ Though small in themselves, they are great reducing agents, striving to bring about that equilibrium so necessary to perfect harmony in the organic world. They hasten decay, tear down the accumulating rubbish around us, and allow the elements thus liberated to pass again into the cycle of ceaseless activity and growth. ”

“ Circumstances obtain in the flowering plants which render mobile bodies like spermatozoids worthless as a means of fertilization. In many cases the male element needs to pass from one tree to another, and even from one country to another. The first observed result of fertilization is the formation of the suspensors, mentioned under Selaginella. At the lower end of the suspensor the young plantlet is formed with its one or more small seed-leaves and a short root and stem. In this growth the food-material in the endosperm is frequently entirely exhausted. ”

“ The Prevention of the Rot.—Now that the life-history of the fluke is known, it is not difficult to comprehend the conditions necessary for its existence. There must be: (1) Fluke-eggs on the ground; (2) wet ground or water during warm weather— (3) the snail Limnæus truncatulus (?) ; and (4) sheep allowed to feed upon the infested ground.
Under the first condition it may be said that wherever fluked sheep are kept we shall have fluke-eggs. In some districts flukes are always to be found, and where the conditions are the most favorable a sudden outbreak may be expected. ”

“ With such multitudes of germs produced we need not wonder that the sowing for moulds should be natural and complete. The light which the microscope throws upon this subject makes it unnecessary to resort to "spontaneous generation" to account for the almost certain growth of mould when the proper conditions are combined. ”

“ These many cases of a seemingly spontaneous growth have led to another definition of the plant-unit which is formulated briefly as follows: A plant individual is that smallest part that can grow when separated from its former place in a plant community, and given the fitting conditions for growth by itself. ”

“ The first generation proceeds directly from the spore, and consists of a simple green expansion which is short-lived and very small, not usually exceeding half the size of a small finger-nail. This prothallus, as it is termed, has small, root-like hairs which fix it to the earth or elsewhere. The prothallia are to be seen in large numbers on the sides of flower-pots in neglected greenhouses. Each little green scale is a young fern-plant during its sexual generation. The male and female organs are much the same shape as those of mosses. ”

“ When the season comes and the soil is ready, the farmer knows he must sow the seed, or he cannot hope to reap a harvest. So it is with the moulds: to the conditions for growth there must be added the germs of life, or no mould will be produced. ”

“ The bones of a recently-killed dog proved to be very well adapted for the growth of the largest-known species of mould. Those who have a passion for scientific names may call it Phycomyces nitens. It is one of the few moulds which grow on oil or oily substances, and is so filthy in its habits as to flourish in the sewers and cesspools of cities. ”

“ At a is a much enlarged view of one of these naked heads of spores, and another where the spores have mostly fallen away. As the fruit-stalks grow old they break down in every way, giving the appearance of a forest over which a tempest has passed. ”

“ The production of the fluke-eggs may be prevented by killing the sheep so soon as they are found suffering from the "rot." If there is a suspicion that a flock is attacked, one of the members exhibiting the strongest signs may be killed and its liver examined. If rotted sheep are kept, they should be on dry ground, where the fluke-eggs can not fall on wet land, or be swept into brooks by the descending rains. The manure of infested sheep should not be placed on wet land, and the livers of rotted sheep ought to be destroyed or deeply buried. ”

“ If a pasture has been flooded, it is well to use the lime to destroy snails which may have been brought down from infested areas.
If there is suspicion that a lowland is likely to give the "rot" to sheep, the best plan is to not allow them to feed upon it. In seasons of excessive moisture it may be impossible to keep the flocks on dry pastures. Salt, as a preventive, may be used in a second way. It acts injuriously upon the germs when fed to the sheep, and it also improves the general health of the animal. ”

“ When the plant is ripe a black elastic coat covers the spore-case, which slips partly off when the spores are discharged from below. There are several species growing on various substances, which have the general structure of Fig. 6.
We now come to the most common of all the members of the group of moulds: the blue mould of cheese, bread, and almost every article of food. In its diet, it does not confine itself to those things found in a well-stocked pantry, but will flourish on old boots and other articles of clothing when they are left for a few days in a warm, damp place. ”

“ In structure and function these pollen-grains are almost identical with the male spores of higher cryptogams. The embryo sac is more or less surrounded by the substance of the parent-plant, and develops within itself a prothallus of small size which is known as the endosperm, and is a store-house of nourishing matter for the young embryo. One or more cells form the homologue of the archegonia in higher cryptogams with its female germ. The male cell or pollen-grain no longer develops a number of mobile, fertilizing antherozoids or spermatozoids ”

“ The fluke-disease alternates between a kind of snail and the sheep. One sheep can not contract the "rot" directly from another member of the flock, and one snail can not take it from its neighbor. The sheep drop the eggs, and hatching, the embryos, find the snails, and the snails after nursing the young flukes through three generations distribute the tadpole cercariæ, which convey the infection back to the sheep, and it there inhabits the liver and causes the disease. ”

“ We thus see that the alternation of generation, viewed in the light of its presentation among mosses and ferns, practically disappears in the higher flowering plants. The sexual generation is so reduced and merged with the asexual that the two seem to become one, and, were it not that the gradual simplifying of this generation may be traced, it would not be thought to exist. ”

“ On the conspicuous moss-plant, arising from such small beginnings, the sexual organs are borne. They are usually produced in clusters at the ends of the leafy axes. Most mosses have only one sex represented in a single tip, and some species have the separation of the sexes so complete that a plant bears only one kind. ”

“ As we pass a little higher in the scale of vegetable life, it is found that though the cells are associated together in filaments, or laminæ, they are, in most respects, very independent, losing only a trifle of their originality by being associated in the simplest form of a community. ”

“ The idea of the identity of the individual among the more complex and perfect forms of existence in the two kingdoms of life may be dismissed, because the methods of propagation in the two are far from the same. ”

“ The pistil is the central part of the flower, and around which all the work of propagation converges and the labors of the year culminate, and from which the new individuals, the seeds, go forth to develop into free and independent colonies. ”

“ The coils grow closer together as the filament increases in size, until a hollow cylinder is formed. From the same threads below the base of the coil one or more processes grow out which are the male organs, and, where one of these threads reaches the tip of the female coiled filament, that organ is fertilized. From this time on, this body undergoes various and complicated changes, which finally result in a bright-yellow spherical body (Fig. 13, f) , which consists of a thin wall inclosing a large number of sacs, g, each of which contains eight spores. ”

“ This young lateral bud, which is frequently so small as to be unseen by the naked eye, is, in fact, the vitalized, undeveloped stem that is to increase in size if growth takes place. The writer is of the belief that in this growing-point the individuality of higher plants should be located. If there are two buds upon the phyton, it seems proper to say there are two individuals, as there are two distinct points of growth, and two branches may result therefrom. ”

“ Other plant individuals devote their energies to the production of new plant-units, namely, the stamens—the male units of the flowers—which perish as soon as their transient but important work of fertilizing the pistil is accomplished ”

“ After this mould, which is of slow growth, has run its course, the bread seldom produces any other forms, and for further study a new culture must be made.
In Fig. 9 is given a mould which makes its home on decaying herbage, and is found to perfection in old waste-heaps where weeds and other green matter have been deposited. So common is it, that a culture is more a matter of convenience than necessity. The fruit-stalks are upright, considerably branched at the top, with the spores borne in bunches at the ends of the filament. ”

“ As we pass upward in the scale, the differentiation increases, and there is a consequent division of labor, some of the cells being devoted to one kind of work, while others engage in a special labor for the community. ”