20 MOSSES AND FERNS chap. 



cell-divisions occur. Often the formation of a germ-tube is 

 suppressed and a cell surface or cell mass is formed at once, 

 and all these forms may occur in the same species. The 

 germination only takes place when the light is of sufficient 

 intensity, and the amount of light is a very important factor 

 in determining the form of the young plant. Thus if the light 

 is deficient, the germ-tube becomes excessively long and slender, 

 and divisions may be entirely suppressed. An excess of light 

 tends to the development at once of a cell surface or cell mass. 

 In the simpler thallose forms the first few divisions in the 

 young plant establish the apical cell, and we cannot properly 

 speak of the gametophore as arising secondarily from a 

 protonema ; in other cases, however, the young plant does arise 

 as an outgrowth or bud from a protonema, which only rarely 

 has the branching filamentous character of the Moss protonema. 



Classification of the Hepaticae 



The Hepaticae are readily separated into the two following 

 well-marked orders : 



Order I Marchantiales. 

 Order 11. Jungermanniales. 



The following diagnoses are taken, with some modifica- 

 tions from Schiffner ((i), p. 5) : 



Order I. Marchantiales. 



Gametophyte always strictly thallose, composed of several 

 distinct layers of tissue, the uppermost or chlorophyll-bearing 

 cells usually containing large air-spaces. The dorsal epidermis 

 usually provided with pores, ventral surface with scales ar- 

 ranged in one or two longitudinal rows. Rhizoids of two 

 kinds, those with smooth walls, and papillate ones ; sexual 

 organs, except in the lowest forms, united in groups which 

 are often borne on special stalked receptacles. The first 

 divisions of the embryo are arranged like the quadrants of a 

 sphere. Sporogonium either with or without a stalk, and all 

 the inner cells forming spores, or some of them producing 

 elaters. No columella present. 



