272 WILLIS : MORPHOLOGY OP THE PODOSTEMACEyE 
passed in ascending the rivers. The same fact has been 
noted by Goebel (13) in the rivers of British Guiana, and 
my own work described in the preceding paper inclines me 
to believe that the same is the case in the rivers of India 
and Ceylon. 
The latest and most complete account of the family has 
been given by Warming in a beautiful series of monographs 
(42), with which we shall now proceed to deal. In the first 
he describes Podostemon Ceratophyllum, Michx., found in 
the United States, and two South American species of 
Mniopsis, showing that the mature plant consists of a thin 
filamentous creeping “ root,” closely attached to the rock, 
often by special organs which he terms haptera, exogenous 
outgrowths of the root or stem, very common in the whole 
family. They are sensitive to gravity and contact, grow 
downwards to the rock and spread themselves out on it, 
being fastened very firmly by the development of rhizoids 
or by a gummy secretion. The creeping root may be termed 
the tliallus ; it bears a more or less aborted root-cap, is green, 
and takes part in the work of assimilation. From it, at 
regular intervals and in acropetal succession, are developed 
secondary shoots, arising endogenously in the tissues of the 
root, and emerging and growing upwards through the water. 
These shoots have a complex morphology of their own, and 
ultimately bear the flowers, usually terminal on the branches, 
which themselves arise in the under axils of dithecous leaves 
(leaves with a stipular outgrowth on the under as well as on 
the upper edges). During the vegetative season the whole 
plant is submerged, but as the water falls the flowers 
emerge. 
In his second paper, Warming describes two new types 
of form. The first occurs in the South American Castelna- 
via princeps, which has a thallus composed of combined 
shoot structures, united to form a flat creeping body bearing 
leaves on the margins (I, c., pi. XIII.), and with the flowers 
emerging from small cavities in the thallus during the dry 
season when the water-level is low. The second occurs in 
