PLANT 598 group above orders is usually the division, of which in dicotyledonous plants there are three, viz., the polypetalous, monopetalous, and apet- alous divisions ; and in monocotyledons there are similar divisions founded on the relations of the floral envelopes to the ovary. Within these divisions the families are grouped in series and cohorts or alliances, bringing to- gether those most nearly related in structure. Plant Distribution. The distribution of plants upon the surface of the globe (geograph- ical botany) forms a distinct and important department of botanical science, which can be merely glanced at here. The study of the dis- tribution of plants as we now find them in- volves not only a knowledge of the require- ments of each species, but of the geological and geographical changes which preceded the pres- ent epoch, and the various natural and artificial agents which have affected the distribution. In treating of germination it has been stated that while seeds will vegetate at a quite low temperature, and may endure a very high one, yet there is a certain point most favorable to each kind of seed. It is so throughout the whole life of the plant; a species may main- tain its existence in an unnaturally low or high temperature, yet there is a degree of heat most favorable for each to perform its functions of vegetation and seed production ; hence tem- perature is one of the chief agents in deter- mining the distribution of plants. It is a well known fact in physical geography that lati- tude alone does not govern the temperature of a locality, but that altitude must be taken into account; on the Alps the harvest is 17 days later for each 1,000 ft. of altitude, and a similar rule holds elsewhere ; it is a matter of common observation that as we go south in our own country, if we would find the com- mon plants of the northern states in Georgia or North Carolina, we must seek them in the mountains. In general terms it may be said that each species requires a certain number of days to complete its career of vegetation and reproduction, and that during these days it re- quires a certain mean temperature ; the mean temperature multiplied by the number of days gives the 'sum of heat the plant demands for its development; within certain limits, if the temperature be lowered, the time must be pro- longed, or if the mean temperature of the days is higher, their number may be lessened ; hence we find vegetation in high latitudes often re- markably rapid, and though the south of Eng- land is nearly 10 south of Upsal, Sweden, the wheat harvest in both localities takes place at the same time. As a general rule a plant will flourish, so far as temperature is concerned, wherever the sum of heat is sufficient for its needs ; it is not necessary to state here the ex- ceptions to this. Another important condition influencing plant distribution is moisture ; the plants native to arid regions, such as the ele- vated plains of Arizona and New Mexico, are especially adapted in their structure to resist a drought of many months, and would die if taken to regions of frequent rains, as quickly as would plants from inoister localities trans- ferred to these plains. Certain plants that live only under the shade of trees are not found in treeless regions, nor are certain others met with away from the influence of the sea. Peculiarities of soil have also their influence on distribution, and there is as much difference between the plants of sandy and heavy soils and of calcareous and granitic ones as between those of swamps and dry hilltops. Some nat- uralists hold that all the plants of a particular species, as at. present recognized, are descended from a single parent individual created " in the beginning;" others that there were several dis- tinct but widely separated "centres of crea- tion;" and others, with Darwin, that species as we now know them were derived from other and simpler "primordial forms." But whatever may have been the " origin of spe- cies " now upon the globe, the fact must be admitted that they have been transferred from the localities they occupied at first and become naturalized in others, for the distribution of plants is still going on. While according to De Candolle no single flowering plant is really a cosmopolite, he enumerates 117 which ex- tend over more than one third of the earth's surface, and 18 which are found over one half of the globe. In contrast with those widely distributed species, there are many of very limited area; the Kerguelen cabbage, found only on the small island which gives its name, is an instance of this ; and again plants of the same kind are found in localities so widely separated as to make it difficult to trace them to a common origin : schizcea pusilla, a very local fern in the pine barrens of New Jersey, was thought to be peculiar to them until it was found in New Zealand; and more than one of our rare plants in the northern states is found wild elsewhere only in the Himalayas or in Japan. Many of the natural methods of disseminating plants are now at work, and it is inferred that they were in operation in early times. The seeds of some plants are carried to long distances by ocean currents, though the number of seeds which can resist the action of sea water is found to be comparatively small ; rivers do much more in the way of dis- tributing seeds, as do casual mountain torrents ; birds may carry aquatic plants long distances, attached to their feet, and birds of passage perform an important part in dissemination, voiding the undigested seeds at a great distance from the point where the fruit was eaten ; quadrupeds do their share of the work, and many seeds and fruits are provided with hooks which allow them to be attached to the hair and skin of these animals. Man performs his part in the distribution ; many plants (espe- cially in the tropics) have escaped from cul- tivation and become so thoroughly natural- ized as to appear as natives, and the weeds of agriculture are to be found in every set-
