LEAVES OF THE HELOBIEAE’ 
AGNES ARBER 
(WITH PLATE 1) 
Introduction 
In a paper published in 1918 (1), the phyllode theory of the 
monocotyledonous leaf was discussed in general terms, and in 
subsequent articles in this and other journals (2, 3, 4, 5) attempts 
have been made to trace the results of applying this theory in 
various special cases. In the present paper, it is proposed to 
study the leaves of the Helobieae, to see how far the phyllode 
theory will help toward interpreting them. I am indebted for 
material to Professor OSTENFELD of Copenhagen; to the Director 
of the Royal Botanic Gardens, Kew; to the Keeper of the Depart- 
ment of Botany, British Museum (Natural History); and to the 
Superintendent of the Cambridge Botanic Garden. 
The Helobieae of ENGLER consist of seven families of water or 
marsh plants. Their common characters are difficult to define, 
but they are united by a macropodous embryo, and on the whole 
they appear to form a fairly coherent group. The seven families 
will be considered individually, and then the general conclusions 
drawn. Since I believe that the Alismaceae and their allies include 
the less specialized types within the cohort, these families will be 
discussed first, instead of following ENGLER’s sequence. 
Alismaceae 
The literature on the protean leaf forms of the Alismaceae 
has been summarized elsewhere (6). The point to emphasize now 
is that the leaves of this family fall into three categories. 
1. Leaves with a sheathing base and a limb, more or less radial 
in form and phyllodic in anatomy.—This form of leaf is rare in 
the family, but is found in the Sagittarias of the S. teres group, to 
which S. isoetiformis Smith and S. teres Watson belong. Fig. 1 
This paper represents part of the work carried out during the tenure of a Keddey 
Fletcher-Warr Studentship of the University of London. 
31[ [Botanical Gazette, vol. 72 
