36 BOTANICAL GAZETTE [JULY 
Hydrocharitaceae 
In the Hydrocharitaceae all three types of leaf which have been 
‘considered are found. In Siratiotes and Enhalus there is no blade, 
and the occurrence of inverted bundles (13, 19) gives a phyllodic 
aspect to the anatomy. These leaves may be regarded as equiva- 
lent to that of Butomus, but in Stratiotes there is no sheath, although 
this region is developed in Enhalus. Hvydrocharis has a leaf with a 
stipulate sheath, a petiole, and a pseudo-lamina, while the leaves 
of Vallisneria and Thalassia are similar to the ribbon leaves of 
Sagittaria. Vallisneria has a sheath which may easily be over- 
looked, while in Elodea this region is entirely lacking (9). 
Conclusions 
A comparison of the leaf structure in the various families belong- 
ing to the Helobieae shows the repeated occurrence of that leaf 
type in which there is a sheathing base succeeded by a bladeless limb, 
in appearance and structure recalling a petiole. Stich leaves, 
instances of which are met with in six of the seven families, are 
regarded as typical petiolar phyllodes. This simple phyllodic 
form of leaf is most characteristically developed in the Juncagina- 
ceae, where it occurs in all five genera. The leaves in this family are | 
generally more or less radial, except in Tetroncium, where they 
are ensiform and Jris-like. In the other families leaves of approxi- 
mately radial structure occur more or less sporadically, or as 
rarities, except in the Naiadaceae, where they are entirely lacking. 
The extreme reduction of the one genus Naias, which constitutes 
the family, however, makes the absence of such leaves by no 
means surprising. 
In each of the seven families examples of a leaf with a sheathing 
base and flat ribbon-like limb are found. This leaf is regarded as 
equivalent to the more obviously phyllodic type of leaf just dis- 
cussed. Two lines of evidence point to this conclusion: (1) within 
the single species Sagittaria sagittifolia L. transitions, both in 
external form and internal structure, can be found between typical 
ribbon leaves and petioles; and (2) within Cymodocea not only typical 
ribbon leaves (as C. nodosa, fig. 12) and typical petiolar leaves (as 
C. isoetifolia, fig. 11) are found, but also in C. manatorum (fig. 10) 
there is an intermediate link between these two types. 
