Aiiicricaii Species of Marchantia. 215 



tion through external conditions, and he emphasizes the fact 

 that the pores of xerophilous forms can often be more or less 

 completely closed by an increased turgidity of the bounding cells. 

 It is clear from the observations of Schiffner and Goebel that 

 the pores in Marchantia (excepting perhaps in M. macro pora) 

 conform to one general type and that the distinctions relied upon 

 by Stephani are less constant than he supposed. This is espe- 

 cially well seen in M. polymorpha, where the inner opening shows 

 all gradations from a quadrate to a cruciate form and thus 

 exemplifies all three of the conditions upon which the first three 

 of Stephani's types were based. M. polymorpha, however, is an 

 exceedingl)^ plastic species and it is doubtful if any of the other 

 members of the genus exhibit the same wide range of variation 

 in the inner opening. Schiffner's figures of AI. geminata, for 

 example, although illustrating conditions connecting the first 

 and second of Stephani's types, show no approach to the third; 

 while in M. paleacea, according to the information at hand, the 

 inner opening is always cruciate and thus does not deviate from 

 the third type. For purposes of taxonomy, therefore, the writer 

 would still consider it expedient to recognize two types of pore 

 among the American species, the distinctions between the types 

 breaking down in the case of M. polymorpha. In the first type 

 (which includes Stephani's first and second types) the inner open- 

 ing is bounded by three to six cells, the usual number being four, 

 and shows all gradations between a polygon, commonly four- 

 sided, with slightly convex sides and one with strongly concave 

 sides and sharp angles. In the second type (which is the same 

 as Stephani's third type) the inner opening is distinctly cruciate 

 with four rays dilated at the apex or, in the rare cases where 

 the number of bounding cells is less or greater than four, with 

 fewer or more rays. Stephani's fourth type, which does not occur 

 in America so far as known, need not be further considered. 



As an example of the first type of pore M. chenopoda may be 

 selected. In this tlie opening, as pointed out by Voigt, is sur- 

 rounded by about seven rows of cells, shown clearly in cross- 

 section (Fig. 19, C-G), four of the rows usually belonging to the 

 upper and three to the lower series. The walls bounding the 

 opening are either smooth or with a resinous deposit. In the 

 upper series (Fig. 19, A, B) the innermost row is usually com- 

 posed of four narrow cells and the second row of the same num- 



