252 THE PTERIDOPHYTA 



be produced under the conditions of moist soil and 

 air where it can live as a semiaquatic and find the 

 free water necessary to the process of fertilisation by 

 motile free swimming male gametes. The freedom 

 from this necessity is not however attained by the 

 existing heterosporous Pteridophytes, in which the 

 microspores still germinate only on damp soil and 

 the sperms still have to swim to the archegonia in a 

 water film. The necessary step to complete freedom 

 from the semiaquatic habitat is the retention of the 

 megaspore in the megasporangium, where it may 

 germinate and produce the eggs in a sheltered position, 

 withdrawn from the danger of desiccation, and the 

 bringing of the microspores to the megasporangium 

 or its immediate neighbourhood, so that the male 

 gametes produced from them may reach the egg 

 without the presence of external water. This is the 

 step which, as we shall see in the sequel, has been 

 taken by the Seed Plants. 



PRACTICAL WORK. 



(1) Examine plants of Pellia, if possible growing plants. Sketch 

 the form of the band-shaped or crisped green branching thallus, 

 with thick midrib attached to the soil by rhizoids and passing 

 gradually into the thinner wings at the edges. 



(2) Examine a transverse section of the fresh thallus, showing 

 the uniform thin- walled tissue. Note that the chloroplasts are 

 mostly concentrated in the upper (sometimes also in the lower) 

 surface layer of cells. In these small starch grains can be seen 

 if the plant has been well illuminated. The central cells contain 

 large starch grains on the surface of which can sometimes be 

 seen the remains of a chloroplast. From the lower surface of 

 the midrib rhizoids arise, each as a tubular branch of a single 

 cell. 



Draw under the high power samples of the cells 6f the upper 

 and lower surfaces with rhizoids and of the central cells. 



3) Place some of the contents of the ripe spore capsule of 

 Pellia in a drop of dilute glycerine and examine the spores and 



