486 Agnes Arber .— The Phyllode Theory of 
represented in Fig. 17 A and B, p. 483 ; these cases are best compared with 
such Acacias as A . cyclops (Fig. 21, p. 483). 
Figs. 15-20, p. 483, illustrate the essential similarity between iso¬ 
bilateral, equitant leaves belonging to different Families. The main anato¬ 
mical difference between them depends on whether the bundles in the flat¬ 
tened region are opposite one another or alternating. In the former case 
they sometimes fuse to double bundles with a single xylem and two phloem 
groups ( Tritonia , Fig. 15 c, and Tofieldia , Fig. 20 C). If they alternate, 
the thinner part of the flattened leaf may contain* a single series of bundles, 
some with their xylem directed towards one face and some towards the 
other ( Anigozanthos , Fig. 19 B, and Iris sp., Fig. 17 b). In the case of 
Tritonia , a slight complication is introduced by the production of a double 
bundle through the branching of the median strand 1 (Fig. 15 A-c), and the 
development of a wing to the midrib. But these variations are unimportant 
from our standpoint, and do not detract from the essential uniformity of the 
anatomical type. 
A problem which seems to demand a solution is why, in Monocotyle¬ 
dons, we only meet with leaves flattened in the vertical plane in cases 
where the phyllotaxis is distichous. The answer probably is that, in the 
case of a leaf with the broad sheathing base characteristic of a Mono¬ 
cotyledon, vertical flattening is almost a physical impossibility if the leaves 
are arranged in more than two ranks. For if the main part of the leaf is 
flattened vertically, the sheath, at least in its upper region, must necessarily 
be correspondingly flattened, and hence the base of the next leaf can only 
be fitted in, if it be placed exactly opposite to the first. In other words, 
we may say that, amongst Monocotyledons, the development of a vertically 
flattened phyllode is conditioned by a distichous leaf arrangement, while in 
Acacia the narrow attachment to the axis renders the leaf independent of 
any special type of phyllotaxis. The opportunity for the development of 
a vertical phyllode must have frequently occurred among Monocotyledons, 
for distichous phyllotaxy is noticeably widespread in this Class. In addition 
to those Families already enumerated, in which—if we may so express it—ad¬ 
vantage has been taken of the distichous habit to develop a vertical phyllode, 
there are numerous other cases of two-ranked leaves. Distichy is common 
amongst the Amaryllidaceae, Scitamineae, Gramineae, Potamogetonaceae, 
Typhaceae, and Sparganiaceae, while cases occur in the Liliaceae, Juncaceae, 
Hydrocharitaceae, Pontederiaceae, and Centrolepidaceae. We may con¬ 
clude that the association of the vertical leaf with distichy does not in any 
way invalidate the phyllode interpretation; it may, however, seem to the 
supporters of the { concrescence theory 7 in some respects favourable to 
their view. 
A case which might perhaps serve to support the ‘ concrescence 
1 Chodat, R., and Balicka-Iwanowska, G. (1892). 
