The Genus Riccardia in Civile. 105 



seen with especial clearness in the Chilean R. prehensilis (Fig. i, 

 D, E), where every surface-cell (except on the stolons) projects 

 forward in the form of a papilla, thus giving the whole surface a 

 velvety appearance. 



A higher type of appendage is present in cases where entire cells 

 are involved, and an excellent example of this condition is found in 

 R. criocaula. At first sight the surface appears velvety, much as 

 in R. prehensilis, but the appearance here is due to distinct, color- 

 less papilliform cells cut off by cell-walls. On the axis these cells 

 are closely crowded, but on the ultimate branches they are more 

 scattered and show their true nature more clearly. It is of course 

 possible that the continuous layer of cells with papilliform projec- 

 tions found in R. preJiensilis might be derived from the condition 

 found in R. criocaula by crowding the cells together so closely that 

 they coalesced. Unfortunately there is little to support this view, 

 except that the papillae in both species are hyaline. 



Unicellular outgrowths lead to conditions where the cells cut off 

 form more complicated structures involving two to many cells. 

 Multicellular outgrowths of this type are seen in a very irregular 

 arrangement in R. spimdifera, which in other respects is a rather 

 simply organized plant. They reach their highest expression, 

 however, in R. fuegicnsis, where they form longitudinal lamellae 

 on the ventral surface of the thallus and arise in distinct acropetal 

 succession. The appendages of the various Chilean species will 

 be discussed in greater detail in connection with the specific 

 descriptions. 



Between the most highly complicated types of thallus found in 

 Riccardia and the simplest types are all possible gradations ; and it 

 is an interesting fact that an intermediate type of thallus may show 

 a high degree of differentiation in some respects and relative 

 simplicity in others. In R. spimdifera, for example, complex 

 appendages are associated with slight differentiation between the 

 axis and its branches. In R. autoica, as will be shown, another 

 combination presents itself. The thallus of this species (see 

 especially Fig. 7, C) exhibits a vague differentiation into axis, 

 photosynthetic branches and stolons, but the histological differences 

 between these various structures are very slight indeed. 



Although the majority of the species of Riccardia are dioicous, 

 a few are monoicous, and a very few sometimes develop bisexual 



