164 



Practical Plant Biology. 



colourless outgrowths are formed from some of the cells of the 

 filament. These outgrowths possess no chloroplasts and they in- 

 sert themselves between the particles of soil. They are rhizoids. 

 The cells of the filament enlarge and divide. The divisions in 

 the cells of the filament, which were at first only transverse, are 

 now often longitudinal and so the filament is gradually converted 

 into a plate of cells, and at the same time cell division becomes 

 more and more limited to a single cell on the margin and to its 

 immediate segments. This cell occupies a depression in the 

 margin and the growth of its segments confers slowly a roughly- 

 triangular form on the plant. The apex being occupied by the 



spore and the original filament, the 

 growing cells are located in an in- 

 dentation in its base. From the 

 under surface of this triangular plate 

 rhizoids are formed. The plate, often 

 called the prothallus, is a very delicate 

 structure not often exceeding a square 

 centimetre in area. It has a transluc- 

 ent green colour. It remains a single 

 layer of cells thick except immediately 

 behind the growing region where it 

 becomes several layers thick, and a 

 cushion-like structure is formed. 



About the same time as the estab- 

 lishment of this cushion sexual organs 

 begin to be developed. The anthe- 



on the cushion just below ridia are formed first and arise princi- 

 the growing point, and pally on the older narrow parts of the 



:n1^e a r z e oMs S . Cattered Phallus and on the under side of 

 the cushion. The formation of sexual 



organs on the prothallus, which has itself developed from a spore, 

 shows clearly that we must regard the prothallus as a gametophyte. 

 The antheridia are almost globular in form and are attached 

 without a stalk directly to the gametophyte. They are composed 

 of a central mass of about forty-eight minute cells enclosed in a 

 rounded cavity formed by two ring-shaped cells lying one on the 

 other, and one plano-convex cell acting as a lid. The nucleus 

 of each of the small inner cells elongates and assumes a spiral 

 form and the cytoplasm clothes it with an exceedingly fine cover- 

 ing of protoplasm which at one end is produced into a tuft of 

 cilia. The absorption of water causes the two ring-shaped cells 

 of the antheridium to swell up unevenly and to distort the en- 



FIG. 4i.Aspidium filix-mas, 



Archegonia are indicated 



