468 
Agnes Arber .— The Phyllode Theory of 
tally expanded phyllodes, so that, on de Candolle’s theory, the venation of 
the Monocotyledonous leaf ceases to present any problem; it shows pre¬ 
cisely those characters which might have been anticipated from the morpho¬ 
logical nature of the organ. Even the more exceptional types of Mono¬ 
cotyledonous leaves fall into line from this point of view; for among the 
Iridaceae (p. 485) certain isobilateral, equitant leaves, which are not strictly 
parallel-veined, reproduce with great exactness the venation of various 
vertically expanded Acacia phyllodes. 
2. The relation of the 4 Phyllode Theory ’ to the theory of the origin of 
Monocotyledons through adaptation to the geophilous habit. 
In a well-known series of papers, Miss Ethel Sargant 1 set forth the views 
that Monocotyledons are descended from Dicotyledonous ancestors, and 
that the single cotyledon of the Monocotyledon is equivalent to the two 
cotyledons of the Dicotyledon, fused into an apparently single organ. The 
present writer, who is in entire agreement with these theories, wishes to 
suggest, as a minor corollary, that the single seed-leaf of the Monocotyledon 
may, like the foliage leaf, be interpreted as phyllodic ; it will then be re¬ 
garded as representing the fusion of the leaf-bases and petioles of the two 
ancestral seed-leaves , the laminae being absent . 2 The phyllodic interpretation 
of the Monocotyledonous seed-leaf has the advantage of explaining the 
frequency, in this Class, of the slender cylindrical cotyledon, showing no 
differentiation into blade and petiole. A sheathing or winged base is some¬ 
times prominently developed in Monocotyledonous seed-leaves, e. g. 
Tigridia , Colchicum , and Elettariap while this region may even be isolated 
into an apparently distinct organ, as in the coleoptile of the Gramineae. 3 
This remarkable development of the cotyledon-base may perhaps be corre¬ 
lated with the loss of a true lamina. 
To account for the origin of the cotyledonary fusion in Monocotyledons, 
Miss Sargant put forward the further theory that this fusion is due to adapta¬ 
tion to the geophilous habit. The present writer would prefer to formulate 
this view somewhat differently, and to regard tl e fusion of the two ancestral 
seed-leaves rather as a structural modification of unknown origin, whose 
occurrence facilitated the adoption of geophilous life, than as an actual 
adaptation to this type of habit. But, with this reservation, she agrees 
with the essential feature of Miss Sargant’s view—namely, that the ancestral 
Monocotyledon was characteristically geophytic in habit—an idea with 
which the phyllode theory of the Monocotyledonous leaf is thoroughly in 
harmony. A study of the leaves of Dicotyledonous geophytes shows that 
1 Sargant, E. (1903), (1904), (1908), &c. 
2 Where a blade occurs it may be regarded as a ‘ pseudo-lamina ’ comparable with those 
of the foliage leaves; see p. 470. 
3 Sargant, E., and Arber, A. (1915). 
