386 



BOTANY 



Order 2. Marehantiaeeae 



The Liverworts included in this order are much more highly organised, and in 

 many genera they have a decidedly complicated structure. Marchantia polymorphs, 

 found growing on damp soil, may serve as an example. It forms a flat, deeply-lobed, 

 diehotomously-branched thallus, about two centimetres wide, and having an incon- 

 spicuous midrib (Fig. 316, A ; Fig. 317, A). From the under side of the thallus 

 spring unicellular rhizoids, of which some have smooth walls, others conical thicken- 

 ings projecting into the inner cavity. The thallus is provided also with ventral 

 scales, consisting of a single layer of cells. In its internal development a dorsi- 

 ventral structure is also apparent. "With the naked eye it may be seen that the 

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

 forated by a central air-pore leading into a corresponding air-chamber immediately 



Fig. 315. — Marchantia polymorph®. A-C, 

 Successive stages in the formation of a 

 gemma ; st, stalk-cell ; D, surface view ; 

 E, transverse section of a gemma ; a:, 

 point of attachment to stalk ; o, oil cells ; 

 r, colourless cells with granular contents, 

 from which the rhizoids will develop. 

 (After Kny, A-C X 275 ; D-E x 65.) 



Fig. 316. — Marchantia polynwrpha. A, A male 

 plant, with antheridiophores and cupules b 

 (nat. size) ; B, section of young antheridiophore ; 

 a, antheridia ; (, thallus ; s, ventral scales ; r, 

 rhizoids. (Somewhat magnified.) 



below (Fig. 158, A, B). The lateral walls of the air-chambers determine the con- 

 figuration 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 of the chambers, but only in small numbers. The air-chambers 

 merely represent depressions in the outer surface which have become roofed over 

 by the more rapid growth of the adjacent epidermal cells. The intensity of the 

 illumination exercises a great influence on the formation of the air-chambers ; when 

 the illumination is very weak they may not occur at all. The epidermis on the 

 under side of the thallus is formed of one layer of cells. The tissue below the air- 

 chamber layer is devoid of chlorophyll, and consists of large parenchymatous cells, 

 which function as accumulatory or reservoir cells. Small cup-shaped outgrowths 

 with toothed margins, the gemmiferous receptacles or cupules, are generally found 

 situated on the midribs on the upper surface of the thallus (Fig. 316, b). These 



