As we travel away from the equator, the vegetation takes on a simpler aspect. There are more annuals and more herbs. The number of Ferns, Grasses, and catkin-bearing Trees, like the Alder and the Birch, increase. The limited growing seasons make for a more restricted accumulation of tissue. Such tropic plants as have braved the rigours of the colder climates have dwindled much in size. The Castor Oil Tree becomes a humble annual (Ricinus Communis) only three to eight feet in height. Other tropical trees become so small that temperate zone folk tread them under foot.
When we get into the polar regions, all the plants take on a stunted and dwarfed appearance and, in some cases, retire almost entirely under ground. The number of genera and species is much reduced. The Oak, Walnut, Chestnut and Elm are replaced by the hardy conifers. At the point where vegetation becomes almost extinct are dwarf Birches, Willows and polar Blackberries (Rubus Arcticus). The simple Mosses and Lichens mark the last lingering evidences of life.
A curious feature of plant life in the polar regions is the rapid growth which it often exhibits. The summer of the Far North is short but it is one day of intense and blinding light. The sun shines continually throughout each twenty-four hours. By virtue of its stimulating power, plants are able to perform in a few weeks processes of development which take months under ordinary conditions.
It is illuminating to take a single country in a more favoured climate and, as far as possible, trace its plant history. The British Isles, because of their limited area, are a convenient field of study. An investigation of their settlement by plants gives us many hints about prehistoric climatic and geographical changes.
Geologists generally believe that the British Isles were once joined to the mainland of Europe. It was at this time that they were settled by vegetation. Some of this plant life came from Spain and some from southwest France; there was also a Germanic group. The floating ice of the glacial period brought over hardy visitors from the Scandinavian peninsula. A few plant immigrants arrived from North America and landed on the west coast of Ireland.
St. Helena is an isolated volcanic mass built up seventeen thousand feet from the bed of the ocean. It therefore has its own peculiar vegetation, a portion of which is believed to have been evolved on the spot from the one-celled state. According to Sir Joseph Hooker, forty out of fifty flowering plants and ten out of twenty-six Ferns “with scarcely an exception cannot be regarded as very close specific allies of any other plants at all.” Sixteen of the Ferns are common to Africa, India or America and were probably carried there by the wind. Ocean currents also brought other species from Africa.
In 1883, a most interesting thing occurred on the Asiatic island of Krakatoa. A violent volcanic eruption wiped every vestige of life off its surface. When the flow of lava ceased and the earth cooled once more, Krakatoa was to all intents and purposes a volcanic island newly risen from the sea. It presented the exact analogy of a recently created bit of land waiting to be settled by the plants. In 1883, it was as barren as the face of the moon. In 1888, a Mr. Hemsley described its appearance as follows:—
“The first phase of the new vegetation, was a thin film of microscopic fresh-water Algae, forming a green, slimy coating, such as may often be seen on damp rocks, and furnishing a hygroscopic condition, in the absence of which it is doubtful whether the Ferns by which they were followed could have established themselves. Both Algae and Ferns are reproduced from microscopic spores, which are readily conveyed long distances by winds. Eleven species of Ferns were found, all of very wide distribution, and some of them had already become common the fourth year after the eruption. Scattered here and there among the Ferns were isolated individuals of flowering plants, belonging to such kinds as have succulent seed-vessels eaten by birds, or such as have a light, feathery seed-vessel like the Dandelion and a host of others, and are wafted from place to place by the winds.
“On the seashore there were young plants and seeds (or seed-vessels containing seeds) of upwards of a dozen other herbs, shrubs and trees, all of them common on coral islands, and all known to have seeds capable of bearing long immersion in sea water without injury. Among the established seedlings were those of several large trees, and a Convolvulus that grows on almost all tropical coasts, often forming runners one hundred yards in length. There were Cocoanuts also, though none had germinated.”
The farther such an island is from the land, the longer will vegetation take to get established. Darwin found that the isolated islands of Keeling, after thousands of years of existence, contained only twenty kinds of flowering plants.