This section is from the book "The Gardener's Monthly And Horticulturist V26", by Thomas Meehan. See also: Four-Season Harvest: Organic Vegetables from Your Home Garden All Year Long.
The lecturer said we habitually recognize differences in the character of vegetation when we peak of Tropical, Temperate or Arctic plants.
Even the most uneducated mind cannot avoid contrasting the rank luxuriance and vast leaf expansion, which are so characteristic of the equatorial lands, with the hard and dwarfed vegetation of polar or Alpine regions. To explain these differences, some have supposed that each particular kind of plant was created as we find it and where we find it; others believing that all the different kinds of plants have descended from a very few primal forms, and that the progeny has, in the long course of ages, wandered from the original home and changed into a multitude of new forms under the influence of varied climates and soils as it was migrating. There are reasons for either belief, as there are also for a compromise between the two opinions.
That plants do change their location is beyond question. We see it going on before our eyes. The long-tailed and plumed seed of the Virgin's Bower (clematis); the leat-like appendage to the seed vessels of the Linden; the wings to the seed of Pines, Ashes and Maples, are contrivances intended to aid in dispersion of seeds by the wind. The lightness of some seeds associated with a mass of hair often affords a most efficient means of effecting a wide dispersion of some seeds. Illustrating this, we have Thistle and Dandelion down. The seeds of the Milkweed, Cottongrass, Willows and Poplars show the same thing.
Plants may be dispersed by currents of water, which transport the seeds long distances and then land them on such spots as allow their growth and increase. Water plants especially illustrate this mode of dispersion. We may almost regard this as their natural mode. Land plants are often so dispersed by some chance which places their seeds in a current of water. It is quite probable that many islands in the ocean owe their vegetation to such accident. The agency of animals in distributing seeds is very important. There appear to be special contrivances for the purpose of aiding in this. Thus, the hooks and the barbs which are found on the seeds or the seed envelopes of the Cocklebur or the Tick-Trefoil; or the Spanish Needles and the Beggars' Ticks, are all illustrations of this. Some of the commonest weeds of cultivation owe their rapid spread thus to animal agency. Other plants have in themselves the means of their dispersion. The Touch-me-Not and the Arceuthobium, by the elasticity of portions of their seed vessels, throw their seeds to considerable distances from the parent plant. Railroads often disperse seeds widely which have come into the country on imported goods.
Thus, without our designing it, these highways of travel frequently give unwelcome additions to the flora of the regions through which they run.
What prevents all plants from spreading over the entire globe?' There are two reasons - first, unfavorable climate, and second, a preoccupation of the soil may prevent a new comer from gaining a foothold in a land unless specially adapted to the new situation. Before a plant or a seed can begin to grow at all, it is requisite that the air have a certain temperature; before it can flower, a definite increase of heat must be had, and a still further increase before it can ripen its fruit. These temperatures vary for different plants, but appear to be quite constant for the same species wherever found. This being the case, one can well understand the importance of temperature in limiting vegetable distribution. Taking a mountain at the level of the sea, if it be possible, in equatorial regions, one may, by ascending its slope from the base to the summit, pass through the following zones of vegetation: ist, palms; 2d, banana, bread fruit and date palm; 3d, coffee, sugar and cotton; 4th, Indian corn, wheat, grapes; 5th, barley and oats; 6th, birches; 7th, lichens. These zones correspond with those observed in going from the equator toward the poles. Hence, then, one can see that latitude and altitude come to be the measure of each other.
For the western coast of Europe it has been estimated that two hundred and sixty-seven feet of altitude produces as much change in the flora as going north one degree of latitude would do, and in tropical America the same result is gained by an elevation of three hundred and twenty-eight feet. The action upon each other of man and the cereal grains has been reciprocal; for while he has carried them around the globe, they have aided in raising the human race from uncivilized wandering herdsmen to civilized communities, which remain stationary, and hence produce the works of art, the wonders of architecture, and the settled habits upon which high mental character, or great national strength, "in the long run," depend.
For a long time observers were puzzled to explain how certain of the common Northern plants came to be found in isolated points far south of their central home. It appears now to be proven that these plants along with the rest of the Northern flora were driven southward by the advancing mass of ice which covered a large portion of our continent as far south as the 40th parallel, in what is known to geologists as the glacial period. When a warmer temperature came, the icy mass gradually disappeared from the whole region south of Greenland and these polar plants could only here and there find in southern latitudes situations which were cold enough for them to thrive in. So we understand how one of the houseleek group (Sedum Rhodiola), has been found in Labrador, Nova Scotia, Pennsylvania, and in Roan Mountains in North Carolina. The present distribution of plants is but the last chapter of a long history. Sometimes a single species furnishes a paragraph which starts some new problem in the past of our globe.
Thus the Scotch Heather, which is now found in Massachusetts, has also been found surviving in Nova Scotia, Prince Edward's Island, Newfoundland, Iceland and the British Islands; and suggests very strongly a continuity of land in former times between Northern Europe and America. Its present station being mere landmarks left along the route of its migration; just as the early civilizing Aryans in their march have left the traces of their advance in the language of the lands through which they passed.
[This is but an abstract of the last Fairmount Park lecture for the season, and was taken by the Public Ledger of Philadelphia - Ed. G. M].