THE BRYOPHYTA : HEPATICAE, THE LIVERWORTS 397 



long time this tissue, which is termed the calyptra, completely envelops the 

 sporogonium. It is probable that the cells of the thallus contribute to this 

 calyptra, for we find abortive archegonia carried up by its growth and lying 

 on the surface of the calyptra, from which we conclude that rapid cell 

 division has taken place in the tissues adjacent to the developing embryo. 

 At the top of the calyptra the remains of the neck of the fertilized archegonium 

 can still be made out. 



Food for the developing sporogonium is obtained entirely from the thallus, 

 and we find that the cells immediately below the foot of the seta become filled 

 with starch. It is important to realize, however, that the foot never has any 

 real tissue continuity with the cells of the thallus, and we can only assume 

 that food material is absorbed by the foot from the surrounding cells, because 

 the cells of the young seta also contain starch grains. 



After its development the sporogonium remains dormant for several 

 months, enclosed in the calyptra. By February it becomes sensitive to light and 

 it normally elongates in March. The cells of the seta, already formed, elongate 

 very rapidly by zones, from the base upwards, the basal zone elongating 

 most. During elongation the seta bends upwards due to a positive reaction 

 to light. As a result of this the calyptra is ruptured and remains as a torn 



/ 



i 



Fig. 379. — Pellia epiphyUa. Multicellular spores and 

 elaters after discharge from the sporogonium. 



membrane round the base of the stalk. During a period of not more than 

 four days the seta grows to 2 or 3 in. long. It is quite slender, and is pure 

 white and almost transparent in appearance, whilst the capsule at the top 

 resembles a large black pin head. Whilst this elongation of the seta has 

 been going on, germination of the spores has also taken place, and before 

 they are shed from the capsule they may consist of several cells. Each cell 

 contains chloroplasts and a nucleus, so that the spore is, in fact, a minute 

 thallus (Fig. 379). 



