In monocotyledonous plants, as grasses, sedges, orchids, bananas, palms and all lily-like plants, there are vascular bundles with xylem and phloem regions, but the bun dles are scattered through the stem and therefore do not form an exterior ring, and there is no true pith. Moreover, these bundles do not contain cambium, and therefore, the stem does not increase much in thickness and does not have a distinct separating bark (Fig. 22). The fibro-vascular bundles are very evident in the stem of Indian corn, and can be pulled out. There are some commercial fibers produced by plants of the dicotyledonous kind. Manila hemp is from a species of banana, and sisal hemp, from an agave, one of the century plant group ; these fibers are derived from the entire bundle, both xylem and phloem, and this origin probably accounts for their stiffness and hardness and their resistance to abrasion.
Longevity of the plant.
In duration, plants are of extreme types. Some kinds live only a few weeks ; some of the trees live for many centuries. It is customary to classify all plants into three groups as respects duration : annuals, living not more than one year from seed to seed, as the cereal grains and most garden vegetables ; biennials, living two years, usually perfecting seed the second year, as beets and parsnips, common mullein ; perennials, living more than two years, as asparagus, alfalfa, bushes and trees. These divisions are not at all exact, however. Annuals are of longer or shorter life within the year, some maturing and dying very quickly from the seed, as the garden cress, and others requiring practically the twelvemonth. Some plants are annual because they are destroyed by frost, and others because they normally complete their growth : the latter, of course, are the true annuals. Those that would outgrow the year if they had oppor tunity have been called plur-annuals: they are plants that have been taken into a shorter-season year, as tomato, castor bean. Plants that are annual in one region, therefore, may be biennial or perennial in another region. Some plants are appar ently annual although they live from year to carrying themselves over by means of bulbs or tubers, as onions and potatoes : these have been called pseud annuals (false annuals). The mullein, bull thistle and teasel are true biennials, part of the growth occurring one year and the completion of the life-cycle the second year. Certain perennials have been bred by man to be biennials, as the cabbage and probably some root crops. Some of the root crops are really annual, as they complete the full cycle in one season if
started early, as the radish. Whether a plant is biennial is often determined by the region in which it grows. There is the widest range in the length of life of perennials. Red clover is a perennial, but very imperfectly so; some forms of it thrive only two years, although they may persist longer. Most peren nial herbs are at their greatest vigor the second and third years, as the strawberry, and then gradu ally weaken, and sometimes even die before very old, new plants having been formed in the meantime. Gardeners know that the best bloom with pinks and hollyhocks and many other showy perennials is secured from plants that are only two or three years old. Sometimes the renewal is accomplished by dividing the old roots.
Societies of plants.
Since plants contend with each other and since different kinds have been driven into similar places or regions, it follows that certain kinds have come to grow together, forming plant societies or communities. A certain set of plants live together in a swamp, and another set on a hill, another in a meadow, and another set in a cotton field or a corn-field. Certain plants grow under or over other plants : grass and bushes may grow under trees ; corn grows above the pumpkins that are planted with it. Wherever plants grow, they are in societies ; that is, they grow together for certain rea sons,—they are adapted to each other or to the place. Some societies seem to be largely accidental in population, however, and others seem to be governed by definite laws or relationships. These laws of adaptation are very little understood. It is now suspected that there may be positive physiological incompatibility between some kinds, and toler ance, congeniality or even symbiotic relationships between others. Under some kinds of trees, for exam ple, certain kinds of herbaceous plants may thrive and others may perish, even when both are equally exposed to sunlight : it is doubtful whether this difference is to be explained by competition for food or moisture. We do not know why some weeds thrive in a corn-field and others do not. There may be bacterial or other organic relations between some kinds. There may be root-excretions that are hurt ful to some plants and harmless or even useful to others. Perhaps the crop rotations thai experience has found to be useful are dependent in some measure on such vital relationships as these.