II. GENERIC CHARACTERS 



The generic characters of Riccardia are clearly given by Spruce 

 (23, p. 540), Stephani (27), and other writers, especially by those 

 who have taken the tropical, south temperate, and antarctic species 

 into consideration. At the same time it seems advisable to precede 

 the detailed descriptions of the Chilean species by a discussion of 

 the more important features of the genus as a whole. 



From the standpoint of morphology Riccardia is of unusual 

 interest. Its least differentiated species are among the simplest of 

 all thalloid hepatics. In forms like these, of which R. piiiguis 

 may be taken as an example, the gametophyte consists of a .flat, 

 dorsiventral thallus with unlimited growth, in which differentia- 

 tion is reduced to a minimum. In other words the branches, which 

 are invariably lateral, if they develop beyond a rudimentary con- 

 dition, are essentially like the axis and eventually give rise to new 

 thalli with the progressive death of the older parts of the plant. 

 In such a type the entire vegetative thallus carries on the photosyn- 

 thetic function. Its dorsiventrality is expressed first of all by the 

 rhizoids and slime-papillae, which are confined to the ventral 

 surface. The rhizoids arise irregularly, apparently in response to 

 external factors, but the slime-papillae are borne in regular acro- 

 petal succession, a single papilla (in most cases at least) being 

 associated with each of the segments cut off by the "two-sided" 

 apical cell (see, in this connection. Miss Clapp, 6, p. 180). Except 

 by the position of the rhizoids and slime-papillae the dorsiven- 

 trality of the thallus is very slightly indicated. The cells of the 

 upper surface-layer are smaller and greener than the others, but 

 these are the only cells at all differentiated, and the cell-walls 

 throughout the entire thallus are thin or only slightly thickened. 

 Toward the margin the thallus thins out gradually until it is only 

 one cell thick, but the marginal portion is like the rest and shows 

 no indication of a wing or a dift'erentiated row of marginal cells. 



In sharp contrast to these simpler types are those in which the 

 thallus shows a high degree of complexity, not only in outward 

 form but also in histological dift"erentiation. Yet these complicated 

 types retain the rather meager ecjuipment found in R. piiigitis of a 

 dorsiventral thallus with constantly lateral branching, and the 

 majority agree further in lacking appendicular organs in the vege- 



