120 STRUCTURAL BOTANY 



3. THE SPOROGONIUM OR FRUIT 



a. External Characters 



If we examine a fertile specimen of Pellia about 

 February, we can easily recognise the young fruits on 

 the upper surface of the thallus. At this stage each 

 fruit appears as a little dark green ball, about one- 

 sixteenth of an inch in diameter, projecting from the 

 involucre (see Fig. 49, lower part). It is attached by a 

 short thick stalk of a lighter green colour, the bottom 

 of which is tightly fixed in the body of the thallus. 

 If we look at the fruit with a lens we see that the 

 upper spherical part the capsule is partly enclosed 

 in a light coloured membrane which it is just begin- 

 ning to burst ; the capsule, where its surface is 

 exposed, is smooth and glossy. The stalk is called the 

 seta. 



Later in the season, about May, a great change 

 happens. The seta grows with astonishing rapidity, and 

 in three or four days attains a length of perhaps as 

 much as three inches (see Fig. 49). The seta in its 

 elongated condition is of a pure white colour, rather 

 transparent, and bears the dark green, or now almost 

 black, capsule aloft on its top, the whole looking like 

 a thick pin with a round head (Fig. 49, on right). 

 Shortly afterwards the dehiscence of the capsule takes 

 place, by four longitudinal fissures, splitting the walls 

 into four valves, which straighten themselves out, forming 

 a horizontal cross on the top of the seta (see Fig. 49, on 

 left). The spores are then set free. It is not long 

 before the seta collapses, and the whole structure, when 

 once the spores are shed, soon perishes. We will now 



