114 BOTANICAL GAZETTE [FEBRUARY 
plied with a limited amount of water developed fruit. At the time 
when the antheridia begin to develop the gametophyte is under 
favorable conditions for vegetative growth, but is not supplied with 
an abundance of water. The soil is moist and the conditions are 
such as would favor the growth of a terrestrial form like Marchantia. 
The plants seem especially adapted to spend the winter submerged 
and do not perish under such conditions. The fore part of the 
thallus contains very large air cavities and thus the tissue is aerated. 
It is well known that certain higher plants which grow in wet situa- 
tions have large air spaces in the tissue, and GANONG (12) calls 
attention to the fact that those marsh plants which are submerged 
for a portion of the year are able to survive on account of their capacity 
for air storage. About May 1 the older part of the thallus, which 
is narrow and thin, has decayed, and the younger parts, bearing 
the sexual organs, is set free and floats. GARBER points out that 
when land forms are placed upon the water only a small portion 
of the apical end remains above the surface, while the older part 
of the thallus extends into the water and decays. This is true of 
plants taken from the soil in summer, but in the spring when the 
free part of the thallus is thick and contains large air cavities, it 
floats readily. The length of the floating period depends of course 
upon the conditions of the pond. In some cases the plants may 
very soon be carried up around the edge of the pond and deposited 
on the mud, but floating forms are usually found until the ponds 
are almost dry. In the case of ponds which do not become dry 
in summer, both forms would be found. The floating period affords 
an excellent means for distribution. 
When the plants grow upon the soil and are not protected during 
the winter by a covering of snow or water, they are usually killed 
by freezing, but in some cases plants which were brown and seemed 
to be dead produced new thalli from the growing point. The young 
delicate thalli are well adapted to tide over the dry season, because 
they can live with a less supply of water than would be needed by 
the older plants. 
RELATIONSHIP OF THE SPECIES. 
The form usually described as Riccia lutescens should be regarded 
as a ground form of Ricciocarpus natans. Both in the field and in 
