466 
Agnes Arber .— The Phyllode Theory of 
I. The Phyllode Theory from the Standpoint of 
External Morphology. 
i. De Candolle’s ‘Phyllode Theory’. 
T HERE is now a considerable balance of evidence in favour of the 
view that the Monocotyledons are descended from Dicotyledonous 
ancestors. If this theory be accepted, it should become possible to trace 
homologies between the various organs occurring at the present day in the 
two groups, since both these groups are thus regarded as the modern repre¬ 
sentatives of an original common stock. From this point of view, the only 
structure in the mature plant which presents any difficulty is the leaf. The 
typical Monocotyledonous leaf is of a simple, more or less linear, form, with 
a sheathing base and parallel veins: how is such a leaf to be compared with 
that of a Dicotyledon, consisting, in its fullest expression, of leaf-base and 
stipules, petiole and net-veined lamina ? 1 This question has naturally 
attracted the attention of morphologists, and an interpretation, which has 
become known as the ‘ phyllode theory was put forward, with some reser¬ 
vations, by de Candolle 2 not much less than a century ago. According to 
this view, the typical Monocotyledonous leaf does not correspond to the 
complete Dicotyledonous leaf, with its leaf-base and stipules, petiole and 
lamina, but is merely the equivalent of a petiole with a sheathing base. On 
this interpretation, the Monocotyledonous leaf, in spite of the reduction 
which it has suffered, still includes within itself, in many cases, parts derived 
from each of the two developmental regions of the leaf—the ‘ Oberblatt ’, 
which normally produces the lamina and petiole, and the ‘ Blattgrund 5 or 
‘ Unterblatt which gives rise to the leaf-base and stipules; 3 or, to use 
Bower’s 4 terminology, it is derived from the hypodium and mesopodium, 
the epipodium having been lost. It seems to the present writer probable, 
however, that in some cases reduction may have gone still farther, so that 
the leaf-base is alone represented, the leaf thus being derived from the 
hypopodium only. 5 
The phyllode theory is supported by the existence of a number of 
examples among Dicotyledons in which organs not dissimilar to typical 
1 In the Phanerogams, with which in this paper we are alone concerned, the differentiation 
between lamina and petiole has become so firmly established that we are justified in treating these 
two regions as morphological entities. But the fact that this distinction of parts holds good for the 
higher plants in no way affects the possibility that the leaf, as a whole, may be the modern repre¬ 
sentative of a thallus-branch, borne by some ancestor of much greater antiquity than the earliest seed 
plant. See Lignier, O. (1908-9), &c. 
2 Candolle, A. P. de (1827). 
3 Eichler, A. W. (1861). 4 Bower, F. O. (1884). 
5 In the present paper the term ‘ phyllode ’ will be used in a comprehensive sense to include all 
foliar expansions lacking a lamina, whether they are morphologically equivalent to ‘ leaf base + 
petiole ’ or to ‘leaf-base’ alone; the two types may be distinguished as ‘ petiolar phyllode’and 
‘ leaf-base phyllode ’. 
