The Origin of the Tristichaceae and Podostemaceae. 
BY 
J. C. WILLIS, M.A., Sc.D., 
Director of the Botanic Gardens , Rio de Janeiro . 
A S is well known, and will be considered in detail in another paper, the 
place to be assigned to these families in the Natural System has long 
been a matter of dispute. They have no nearly allied family^ unless possibly 
Hydrostachydaceae, which live in similar conditions, and were once united 
to them. For the time being the dispute has ceased, since Warming placed 
them in the same order with the Saxifragaceae, with which they have 
in common a thick placenta and numerous ovules. I may remark, however, 
that I think they are just as near to the Nepenthaceae, which belong to 
another order, and probably to other families as well. It will help consider¬ 
ably in treating this difficult question, as well as in dealing with their 
evolution, if we consider in some detail their probable mode of origin. 
These plants, it may be well to explain, are all water plants with a mode 
of life which in the flowering plants is unique to them and to the Hydro¬ 
stachydaceae. With the exception of one species in the southern United 
States they are confined to the tropics and sub-tropics, but are very widely 
distributed, reaching from Java by India and Ceylon to Egypt and Mada¬ 
gascar, tropical and South Africa, South and Central America, and to 
Mexico and the United States. They live only attached to rocks, usually 
smooth and waterworn, or other firm substratum, in the beds of mountain 
streams, where the water moves rapidly over them, so rapidly that it is often 
full of bubbles of air. If they be placed in standing water they soon die, and 
they are absolutely incapable of existence on a sandy or muddy substratum. 
They cling to the rocks by means of root-hairs, or more often by 
haptera, special adhesive organs of probable root nature, which usually 
appear as exogenous protuberances on root or shoot, bend downwards 
to the rock, and there cling by flattening out and secreting an adhesive 
substance. 
These plants are purely vegetative during the period of high water-level 
due to the rains. As the water drops towards the close of the rains, they 
form their flowers, which open as the air touches them. The seeds quickly 
[Annals of Botany, Vol. XXIX. No. CXIV. April, 1915.] 
