467 
the Monocotyledonous Leaf. 
Monocotyiedonous leaves can be shown to be equivalent to leaf-bases, or to 
both leaf-bases and petioles. Such cases are numerous and familiar, and it 
is unnecessary here to do more than briefly to recall their existence. Those 
instances in which the reduced leaves correspond to leaf-bases, with or 
without stipules, are the commoner. A series of stages can, for example, be 
traced in such a bud as that of the Black Currant ( Ribes nigrum , L.), show¬ 
ing transitions from a protective scale with parallel veins to a normal leaf 
with blade and petiole (Fig. 5 A (i)-(v), p. 474)- Similar transitions can be 
readily followed in the earlier leaves of the long shoots of cultivated Roses 
and in the expanding bud of the Horse Chestnut. The scale-leaves of 
Monotropa , as de Candolle 1 has pointed out, clearly correspond to the bases 
of the petioles in the related genus Pyrola , while the scale-like prophylls of 
Ranunculus Ficaria, L., are also equivalent to leaf-bases (pr. in Fig. 4, p. 474). 
The cases in which petiole as well as leaf-base undoubtedly plays 
a part in the leaf-like expansion are less numerous, but great theoretical 
interest attaches to them in connexion with the phyllode theory. Certain 
species of Oxalis , e. g. O. bupleurifolia , A. St. Hil. (Fig. 3 A, p. 474), show 
every stage in reduction of the lamina, correlated with a blade-like develop¬ 
ment of the petiole. The phyllode, in this case, is horizontally expanded, 
whereas in the numerous phyllodic Acacias it is most commonly flattened 
in the vertical plane ; in one species, however (A. leptospermoides , Benth.), 
the phyllode is described as horizontal, 2 and thus comparable with that of 
Oxalis bupleurifolia. while in others it is not flattened but almost radially 
symmetrical (e. g. Acacia scirpifolia , Meissn., Fig. 1 A and B, p. 474). That 
the phyllode in this genus is truly petiolar is deduced from the series which 
can be traced in certain seedlings between normal pinnate leaves with slender 
petioles—modified leaves with reduced laminae and flattened petioles— 
and, finally, phyllodic expansions with no trace of a lamina. Such a series 
is indicated in Fig. 2 A, p. 474), which represents Acacia neriifolia , A. Cunn., 
but a more complete set of transitional forms can often be found. 
It is a commonplace of every text-book that one of the most distinc¬ 
tive features of Monocotyledons is the parallel venation of the leaves; the 
‘ extraordinary uniformity ’ characterizing the main phenomena of venation 
in the striated type, to which the majority of Monocotyledons belong, was 
emphasized many years ago by de Bary. 3 But no theory hitherto pro¬ 
pounded regarding the origin of Monocotyledons has offered any satisfactory 
explanation of this well-marked character of the Class. To the present 
writer it appears that one of the chief merits of de Candolle’s phyllode 
theory is that it explains the parallel venation of Monocotyledonous leaves 
in a perfectly unstrained way. For parallel veining is one of the most 
obvious characters of Dicotyledonous leaf-bases and petioles and of horizon- 
1 Candolle, A. P. de (1827). 
3 Bary, A. de (1884) 
2 Hochreutiner, G. (1896). 
