430 



BOTANY 



PART II 



comprising exclusively thalloid forms, the Jungermanniaceae, including 

 both thalloid and dorsiventral foliose forms, and in the group of the 

 Haplomitricae, radially constructed foliose forms. 



Order 1. Rieciaeeae (i^°) 



The genus 



Of all tlie Hepatieae, this order exhibits the simplest structure 

 liiccia belongs to this order ; its dichotomously-lobed or cleft thallus forms small 

 rosettes, and grows on damp or marshy soil. Biccia nutans is found floating, like 

 Duckweed, on the surface of stagnant water. Riccia fixiitans, on the other hand, 

 lives wholly submerged, and has narrow, more profusely branching, thalloid seg- 

 ments (Fig. 369, A). These two aquatic species can, however, grow on marshy soil, 



and then form flat rosettes (Fig. 

 369, B). The Riccias are provided 

 with fine rhizoids springing from 

 the under side of the thallus, and 

 possess, in addition, a row of trans- 

 versely disposed ventral scales, 

 consisting of a single layer of cells, 

 Mliich also assist in the absorption 

 of nourishment. Both organs are 

 wanting in the submerged form of 

 Pdccia fluitans. 



The antheridia and archegonia 

 are sunk in the surface of the upper 

 side of the thallus. From the fertilised egg-cell is developed a spherical sporo- 

 gonium which has no stalk. The wall of the sporogonium consists of a .single 

 layer of cells ; it becomes disorganised during the ripening of the spores, which 

 are eventually set free by the rupture and disintegration of the venter and the 

 surrounding cells of the thallus. 



B 



Fig. 369. — A, liiccia fluitans ; submerged floating form. 

 B, Riccia natans ; lanci form. (Nat. size. B, after 



GOEBEL.) 



Order 2. Marchantiaeeae (^^^) 



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

 many genera they have a decidedly complicated structure. Mardiantia ^^olymoiyha, 

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

 lobed, dichotomously-branched thallus, about two centimetres M'ide, and having an 

 inconspicuous midrib (Fig. 371, A, Fig. 372, A). From the under side of the thallus 

 spring unicellular rhizoids, some of which have smooth walls and serve mainly 

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

 cavity ; these peg-rliizoids serve for water conduction. 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. AVith 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 corre- 

 sponding air chamber immediately below (Fig. 170, A, B, and p. 165). 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, eacli tier comprising four cells. Numerous 

 short filaments, consisting of rows of nearly spherical cells containing chloi-ophyll 



U 



