426 



BOTANY 



Order 2. Marehantiaeeae ("") 



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

 many genera they have a decided complicated structure. .V" /<// />////" //.//v//"-//'/"'. 

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

 lohed, dichotomously-branched thalltis, about two centimetres wide, and having an 

 inconspicuous midrib (Fig. 381 A, Fig. 382 A). From the under side of the thallus 

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

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

 cavity ; these peg-rhizoids 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. With the 



Fio. 380. Mitii-li'iiitni i*ilyiii<iriili<i. A-C, 

 Successive stages in the formation of a 

 gemma; st, stalk-cell; /). surface view; 

 K, transverse section of a gemma ; B, 

 point of attachment to stalk ; , oil cells : 

 r, colourless cells with granular contents. 

 from which the rhizoids will develop. 

 (After KNY, A-C x 275 ; D-K x 65.) 



Fl<;. 3)S1. Man-kit iiliii jxi///w(i/^(7i". .1, A mule 

 plant, with antheridiophores and gemma-cups >> 

 (nut. si/e) ; /;, section of young antlieriilio])hoi-e : 

 a. antheridia : /, thallus; s, ventral seal- 

 rhizoids. (Somewhat magnitled.) 



naked eye it may be seen that the Upper surface of the thallus isdivided into small 

 rhombic areas. Each area is perforated by a central air-pore leading into a corre- 

 sponding air-chamber immediately below (Fig. 160 A, B). 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 functi<m> "I 

 assimilating tissue. Chlorophyll grains are found also in the walls and roof of the 

 chambers, but only in small numbers. The air-chambers 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 serve as storage cells. 



