456 MR. H. B. GUPPY : PLANT-DISTRIBUTION 
subdivisions of the Dicotyledons with respect to this feature in distribution. 
Sympetale and Monocotyledons are distributed in the same proportions in 
both worlds, as respects their families, although the first holds only 2 per 
cent. of aquatic families and the second as much as 30 per cent, Yet it is 
true that the large proportion of aquatic families exercises an influence in 
determining the distribution of Monocotyledons ; but, as is established below, 
that influence is mainly concerned in curtailing their latitudinal extension 
and affects but slightly, as already shown, their response to the bi-cleavage 
of the land, as indicated by the proportion of families existing in both 
hemispheres. 
Although the Monocotyledons and the Dicotyledons are at one in the 
similar responses of their families to the great cleavage of the land, they 
differ much in the responses made by their families to the differentiation of 
the latitudinal climatic zones. Whilst, as shown in Tables IV. and V., both 
classes hold about the same proportion of exclusively or mainly tropical 
families, 98 and 59 per cent. respectively, yet in the case of the Dicotyledons 
nearly all of the residue are either mainly or exclusively temperate, while 
with the Monocotyledons two-thirds of the remainder are fairly well shared 
between the tropical and temperate zones. Itis thus evident that as regards 
the differentiation or separation of temperate floras from the original tropical 
floras, the Dicotyledons are in a much more advanced stage than the Mono- 
cotyledons. The equal sharing between the tropical and temperate zones of 
a family originally tropical represents the first stage in the detachment of a 
temperate family. The appropriation of a family by the temperate zones 
represents the last stage in the detachment of a family from its original 
abode in the tropics. This last stage has been attained by 21 per cent. of 
the families of the Dicotyledons and by only 7 per cent. of those of the 
Monocotyledons (Tables IV., V.), the last-named having lagged behind the 
Dicotyledons to a marked degree as regards the differentiation or detachment 
of temperate floras. The contrast may be stated in another way. Thus, 
whilst 34 per cent. of the families of the Dicotyledons are either temperate 
or mainly temperate, the proportion for the Monocotyledons is only 14 per 
cent. This may be due to the greater prevalence of aquatic families among 
the Monocotyledons. Here the proportion is as much as 30 per cent., that 
for the Dicotyledons being under 6 per cent. (Table V.). The explanation 
would be that aquatic conditions present a much smaller contrast between 
the temperate and tropical zones than is offered by those of land plants. 
The conclusions to be drawn from the behaviour of the great. plant-groups of 
the Angiosperms.—Although the present arrangement of the main land- 
masses and of the oceans is largely ignored by the great plant-groups, the 
response becomes more and more evident as we go down the differentiating 
scale. It goes without saying that in whatever way we split up the Dicoty- 
ledons, whether in two or three or four groups, all the primary groups of 
