ern hemisphere. In examining the dis- 

 tribution 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 Monocotyle- 

 dons, 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 un- 

 favorable for Monocotyledons, it is hard 

 to say. Of course in these cases the world- 

 families already mentioned are repre- 

 sented. 



The other great division of Angio- 

 sperms is known as Dicotyledons, which 

 include such forms as our common for- 

 est trees, buttercups, roses, peas, mints, 

 sunflowers, etc. As there are about eighty 

 thousand of these Dicotyledons, it is im- 

 possible 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 ten- 

 dency of the group, some of the facts of 

 distribution are as follows : 



It has been noticed that the Monocoty- 

 ledons 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 Coni- 

 fers, which only develop tree types. With 

 the coming of the Dicotyledons, there- 

 fore, the vast temperate and boreal re- 

 gions presented a particularly favorable 

 field, which they have entered and taken 

 possession of. This vast group is promi- 

 nently adapted to living in the unoccu- 

 pied 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 evi- 

 dent that there is no such difference be- 

 tween 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 west- 

 ern forms are separated from one anoth- 

 er, not merely by oceans, but by temper- 

 ate and boreal lands. In the case of Di- 

 cotyledons 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 num- 

 ber 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 geo- 

 graphic 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. 



44 



