406 WILLIS : MORPHOLOGY OF THE PODOSTEMACEÆ 
of the stem is like that of other highly adapted water plants, 
but requires study in detail. 
The primary axis in most forms shows little sign of 
dorsiventrality, unless in the excentricity of the bundles. 
In Lawia the curious lateral growth, of or from the primary 
axis, resulting in the formation of the thallus, requires 
further investigation. In Podostemon subulatus the primary 
shoot shows a certain unequal symmetry in the way it forms 
an upper side by the curving over of the leaves, while 
in Farmeria metzgerioides there seems to be an actual 
dorsiventrality in the very cotyledons, which are diverted a 
little towards the lower side, and slightly unequal. None 
of the PodoBtemaceæ as yet investigated have any developed 
primary root, as indeed is only to be expected from their 
mode of life. 
The ThalBys and Secondar^f Sheets. 
These organs are best considered together, as they are 
essentially connected in the general ecology and morpho- 
logical construction of these plants, and not infrequently 
also, as in Lophogyne, the secondary shoots themselves are 
developed in thalloid forms, making what we may perhaps 
call secondary thalli. Under the general term thallus or 
primary thallus we may include the creeping dorsiventral 
organ which is developed from the primary axis and itself 
bears endogenous secondary shoots. These, in the Indian 
forms, are not themselves thalloid in structure, but are simple 
leaf-bearing axes, ultimately floriferous in all but Lawia. 
Phylogenetically considered, the thalli found in the different 
genera of the order are of different morphological value. It 
is evident that the thallus of Lawia is not homologous with 
the other Indian thalli, and it is doubtful if it is exactly 
equivalent to any of the other thalli found in the order. It 
is evidently of shoot nature, if we judge only by the upper 
side, which bears leaves, and whose growing points end in 
flowers, but, on the other hand, we have to note the curious 
growing point with its root-cap-like lower edge, and the 
