330 Arber . — The Leaf Structure of the Iridaceae , 
more primitive. There is no single species of Iris 1 covering the range of 
the genus, but the rhizomatous Section Apogon, with its ensiform leaves — 
to which the widely ranging I. Pseudacorus, L., and I. foetidissima , L , 
belong — taken as a whole, covers the entire area of distribution of the genus, 
from the Pacific Coast of North America in the West, to China and Japan 
in the P'ast. The members of this Section far outnumber those of the 
other subdivisions of the genus, and their numerousness, as well as the 
non-bearded perianth segments, maybe taken as indications of the antiquity 
of the type. Ensiform leaves are also typical of the other large rhizomatous 
Section, Pogoniris, as well as of several smaller Sections of the genus. The 
bulbous Sections, Tetragonae , Xipliium , and Juno , on the other hand, have 
a non-ensiform leaf. There seems to be some degree of probability that in 
the Iridaceae the rhizome may have preceded the bulb. Among the 
Liliaceae the two related tribes which show the most generalized characters 
- — Melianthoideae and Asphodeloideae — are prevailingly rhizomatous ; it 
appears likely that it is from some such Liliaceous stock that the Irids 
have arisen. That the bulbous Irises represent a specialized and relatively 
late development is also indicated by the fact that, in comparison with the 
rhizomatous types, they are confined to relatively restricted areas. The 
Tetragonae , whose leaves may, as I have shown, be regarded as a variant 
upon the ensiform type, are centred about Asia Minor and the Caucasus ; 
the Xiphiums, which show transitions towards leaf-base leaves (p. 312), belong 
to the Iberian and North-west African regions; the Juno Irises, whose 
leaves are reduced to leaf-bases pure and simple (p. 314), though they are 
found from the Mediterranean to North-west India, cannot compete in 
range with the ensiform Apogon Section. 
The Irises are confined to the Northern Hemisphere. That they 
represent a very ancient stock which has had a long time to differentiate, 
is indicated by the fact that the genus has a wider area of distribution than 
any other within the family : .another point which also suggests the great 
age of the Iris type is that, with the one exception of /. selosa, Pall., 2 all 
the American species are endemic. In Africa, south of the Atlas Mountains, 
Irises are absent, and their place is taken by the closely related genus 
Moraea , one species of which extends into Australasia. In the Moraeas 
we find typically ensiform leaves resembling those of the majority of Irises, 
as well as reduced leaves recalling those of the Xiphium Section. These 
two genera — Iris and Moraea — together range over almost the whole world 
except South America. The place of the Iridaceae, in tropical and sub- 
tropical America, seems to be taken by the Maricineae, of which Marica — 
1 On the distribution of members of the genus Iris, see Dykes, W. R. (1918). 
2 See Dykes, W. R. (1913), pp. 93 , 94 . This Iris , of which a variety of forms are known 
to come true from seed, occurs in Northern Asia and passes into America through Kamtchatka and 
Alaska. It is thus possible that it entered America in relatively recent times, though sufficiently; 
long ago to have become differentiated in that continent into a variety of distinct types. 
