Lecture XX. 169 



be analogous. Thus homology is a relational similarity while 

 analogy is a similarity of function of parts. 



PRACTICAL WORK. 



Sketch the under surface of a fertile, or sporangium-bearing, pinnule of a 

 sporophyll. Observe the indusium and sporangia. 



Make a transverse section passing through the sorus. Sketch, showing 

 the placenta, indusium and sporangia. 



Scrape off some sporangia approaching maturity into boiled water (to re- 

 move the adhering air bubbles) and mount in water. Sketch showing the 

 stalk, annulus, stomium and spores of the sporangia. 



Scrape off some sporangia from a ripening sorus on to a dry glass slide. 

 Observe with the low power. Notice the bending backwards of the annulus 

 and the consequent tearing open of the sporangium. Observe the sharp jerks 

 of the recurved annulus. 



Gametophytes for examination may be obtained by sowing the spores shed 

 from the leaves on sterilised peat mould. The peat mould is placed in a 

 flower saucer, and moss spores, etc., which it might contain are killed by 

 pouring boiling water over it. The earthenware saucer is covered with a 

 glass plate and is placed standing in a shallow glazed saucer containing 

 water. A good crop of gametophytes may be expected in a month or so. 



Take a gametophyte and place it upside down upon a glass slide. 

 Gently remove the adherent particles of peat. Add a drop of water and 

 cover. Observe the colourless rhizoids and the green cells. Examine one 

 of the latter with the high power. Note the numerous chloroplasts and the 

 various stages in their division. Look for antheridia and archegonia. The 

 former may be recognised by their circular outline and stellate cavity. Care- 

 ful focussing will reveal the two ring-shaped cells. Often a mass of sperm- 

 mother-cells may be seen in the antheridium and the beginning of the motion 

 of the sperms may be detected within them. 



The archegonia are less easily recognised, chiefly because they are sunk in 

 the thicker part of the gametophyte. From the surface aspect of the pre- 

 paration only two of the four parallel rows of cells forming the neck are at once 

 visible. A dark brown streak between them indicates the neck-canal leading 

 down to the ovum. Focussing down along this brings the cavity of the 

 venter and the ovum into view. 



" Reconstruct the shape of an archegonium from a series of optical sections 

 taken in the neck and the venter. 



Sketch an older gametophyte with young sporophyte attached. 



