96 
F. Cavers. 
of tissue, so that the thallus comes to consist almost entirely of a 
spongy mass, the compact ventral tissue being reduced to two or 
three layers of cells (Fig. 5). The ventral scales are long and 
narrow and are scattered over the whole underside of the thallus; 
they shew long-continued growth in length owing to the basal cells 
being meristematic. In the land-form, which Lindberg (Bot. 
Centralblatt, Band 6, p. 363) showed to be identical with Riccia 
lutescens (a fact recently discovered independently by Lewis), the 
thallus is of simpler structure and the scales are short and semilunar 
in shape. In the other species of Riccia, the scales are developed 
in a single series, hut they usually become split in the middle, 
giving rise to two lateral rows; in the hygrophilous species they 
soon perish, but in xerophilous species they are large and 
persistent. 
Fig. 6. Riccia natans. A, longitudinal section of thallus, showing 
antheridia and archegonia; B. transverse section of archegonium-bearing 
portion ; C, transverse section of antheridium-bearing portion. After Garber. 
The sexual organs are usually scattered, and sunk separately 
in deep cavities. In some species there is a tendency to the 
