The Monocotyledons are also somewhat famous for the number of air plants which they contain—that is, plants which have sometimes been called “perchers,” because they fasten themselves upon trunks and branches and supports of various kinds, and absorb what they need directly from the air. It is a notable fact that these so-called “perchers” are very much more abundant in the western tropics than in the eastern. An explanation for this is to be found in the fact that the western tropics have a very much greater rainfall; in fact, in the rainy woods of the Amazon region the air is saturated with water, and everything is dripping.
One of the facts in connection with the distribution of Monocotyledons is quite puzzling, and that is the very poor representation of the whole group in the southern hemisphere. In examining the distribution of other groups in the southern hemisphere, it is found that Australia and its general vicinity is prolific in peculiar forms. In the case of the Monocotyledons, however, the Australasian region is the most poverty-stricken one in all the southern hemisphere. Just why the southern hemisphere in general, and the Australasian region in particular, are unfavorable for Monocotyledons, it is hard to say. Of course in these cases the world-families already mentioned are represented.
The other great division of Angiosperms is known as Dicotyledons, which include such forms as our common forest trees, buttercups, roses, peas, mints, sunflowers, etc. As there are about eighty thousand of these Dicotyledons, it is impossible to state anything very definite in reference to the distribution of the group as a whole. Taking the higher forms, however, as representing the general tendency of the group, some of the facts of distribution are as follows:
It has been noticed that the Monocotyledons are massed in the tropics, and that the temperate and boreal regions have been left comparatively free by previous groups, with the exception of the Conifers, which only develop tree types. With the coming of the Dicotyledons, therefore, the vast temperate and boreal regions presented a particularly favorable field, which they have entered and taken possession of. This vast group is prominently adapted to living in the unoccupied temperate and boreal regions. This does not mean that they are not found in the tropics for they hold their own there with the other groups.
Dicotyledons, however, succeeded in working out but three world-families: Composites, to which the sunflowers, dandelions, etc., belong; the Mints; and the Plantains. There are other large families which characterize certain great areas, but they are not world-wide in their distribution.
Another fact, which might indicate that the Dicotyledons have taken possession of comparatively unoccupied regions only, is that they are very poorly represented, so far as higher groups are concerned, in aquatic conditions. It would seem as though the conditions of life in the water had been fairly well taken up by other groups. In looking over the display of Dicotyledons in the tropics of the eastern and western hemispheres, it becomes evident that there is no such difference between the forms of the two regions as in the groups previously mentioned. It will be remembered, however, that in the case of the Cycads and palms, which were used as illustrations, they are restricted to the tropics, and their eastern and western forms are separated from one another, not merely by oceans, but by temperate and boreal lands. In the case of Dicotyledons this is different, for while they are found in the tropics, they are found in the other regions as well, and have better chances for intermingling than the other groups.
This tropical display of Dicotyledons further shows the great prominence of America in the display of forms. This appears not merely in the greater number of peculiar forms and often families which appear in tropical America; but whenever the continents are paired in the display of forms, America is always one of the pair, Asia or Africa being the other member.
It will be recognized from what has been said that the whole subject of geographic distribution is a very extensive one, and that it will be a long time before the important facts are recorded. The importance of the subject rests not so much upon the mere presence of certain plants in certain regions, but it has to do with explaining just why the conditions are suited to the plants, and also just how the plants have come to be what they are and where they are.
John Merle Coulter.
