Lecture XVI. 127 



that different characteristics are dealt out to the different cells as 

 they are formed in the growing region in response to external 

 stimuli, or that these stimuli differentially inhibit or develop 

 potentialities in the dorsal and ventral cells. 



PRACTICAL WORK. 



Sketch both aspects of the thallus of Marchantia polymorpha, magni- 

 fying it 4 to 8 diameters. 



Cut off a thin piece from the upper surface, mount and examine. Sketch 

 the rhomboidal areas and the central pore, using the low power. 



Make a transverse section and mount. This requires to be very thin to 

 show the structure of the dorsal tissues satisfactorily. A sketch should be 

 made with the high power of the green layer, showing the cells round the 

 pore, those of the roof, floor and photosynthetic filaments of an air-chamber. 



Sketches should also be made of the cells of the storage tissue and of the 

 two kinds of rhizoids. 



Specimens showing cupules and gemmae are easily obtained in May and 

 June. The vegetative structure of the thallus may be studied at any season 

 of the year. 



