Batters. — On Schmitziella , 
190 
cells. The mature thallus of Schmitziella thus presents the 
appearance of a membranous veined expansion, the long- 
celled primary filaments representing the veins (Fig. 7). 
At first all the branches of both primary and secondary 
filaments are confined to one plane, but as the formation of 
the thallus continues it sometimes happens that a branch is 
pushed out of its place and creeps over or under the already 
formed network of cells. These displaced filaments, in their 
turn, send out branches, and in this way the thallus becomes 
in places a two- (very rarely a three-) layered membrane. 
Where this occurs all the filaments of the Schmitziella 
are sometimes confined between the same two layers of 
the compound cell-wall of the Cladophora ; but more 
frequently each layer of the thallus of the Schmitziella is 
contained between separate layers of the cell-wall of the 
host-plant. 
The cells of which the thallus is composed are laterally 
united to each other in a very loose manner, and it is some- 
times not very easy to determine to which branch a cell 
belongs, but usually it is no very difficult matter to trace the 
branching. The shape of the cells is most variable, oval, 
oblong, or sickle-shaped cells being most frequently met with, 
but sometimes they are so irregular that it would be useless 
to attempt to describe them. The cell-membrane is always 
thin and delicate, never chalky, as is the case in nearly all the 
other members of the order. 
The reproductive organs are developed in nemathecial sori, 
which are more or less numerously scattered over the surface 
of the thallus, and arise on its upper surface in the form of 
flattened hemispherical protuberances. To form these, the oval 
or oblong cells of the thallus are collected together at various 
spots, and from them, by transverse division, smaller roundish 
cells are cut off either singly or by twos. These smaller cells, 
becoming united more or less firmly to one another, form the 
basal layer of the sorus. By an almost simultaneous upward 
growth of these smaller cells, which themselves develope into 
its component threads, the flattened hemispherical sorus is 
