484 



BOTANY 



PART II 



sporogonia, unlike those of all other Hepaticae, do not ripen simultaneously 

 throughout their whole length, but from the tips downwards, and continue to 

 elongate by basal growth after emerging from the archegonia. The wall of the 



FIG. 442. Anthoceros laevis. 

 sp, Sporogonium ; c, colu- 

 mella. (Nat. size.) 



FIG. 443. Anthoceros Pearsoni. Development of the 

 endogenous antheridium. d, Covering cells ; st, 

 stalk-cells ; a, young antheridium. (After D. 

 CAMPBELL.) 



sporogonium possesses stomata, which do not occur in other Liverworts ; chlorophyll 

 is present in its cells. 



On the under side of the thallus, slit-like openings, formed by the separation 

 of the cells, lead into cavities filled with mucilage. Nostoc filaments penetrate 

 into these cavities, and develop into endophytic colonies ( 101 ). 



Order 2. Marchantiales (" l02 ) 



The Liverworts included in this order in many genera have a decidedly com- 

 plicated structure. Marchantia polymorpha, found growing on damp soil, may 

 serve as an example. It forms a flat, deeply-lobed, dichotomously-branched 

 thallus, about two centimetres wide, and having an inconspicuous midrib 

 (Fig. 445 A, Fig. 446 A). ' From the under side of the thallus spring uni- 

 cellular rhizoids, some of which have smooth walls and serve mainly to attach 

 the thallus, while others have conical thickenings projecting into the cell- 

 cavity (Fig. 31) ; these peg-rhizoids are collected to form a wick-like strand below 

 the midrib. The thallus is provided also with ventral scales, consisting of a 

 single layer of cells. The dorsiventrality of the thallus is further shown by its 

 complicated anatomical structure. With the naked eye it may be seen that the 

 upper surface of the thallus is divided into small rhombic areas. Each area is 

 perforated by a central air-pore leading into a corresponding air-chamber immedi- 

 ately below (Fig. 95 A, ). The lateral walls of the air-chambers determine the 

 configuration of the rhombic areas. The air-pore in the roofing wall of each 

 chamber is in the form of a short canal, bounded by a wall formed of several tiers of 

 cells, each tier comprising four cells. Numerous short filaments, consisting of rows 

 of nearly spherical cells containing chlorophyll grains, project from the floor of the 

 air-chambers and perform the functions of assimilating tissue. Chlorophyll 

 grains are found also in the walls and roof of the chambers, but only in small 

 numbers. The intensity of the illumination exercises a great influence on the 

 formation of air-chambers ; when the illumination is very weak they may not 



