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A TEXTBOOK OF THEORETICAL BOTANY 



embryo, and consists of a single-layered wall surrounding a mass of cells, 

 which is called the archesporium, from which the spores are formed. It 

 is interesting to note that in the divisions of the archesporial cells centrosomes 

 are present, a rare phenomenon in plants. The cells of the seta undergo 

 repeated divisions but remain very short until the capsule is ripe. At its 

 lower end the seta develops a conical foot, with a flange projecting upwards, 

 in outline like a barbed arrow-head, by means of which it is firmly embedded 

 in the thallus (Fig. 378). 



Calyptra 

 Theca 



Unfertilized 

 archegonium 



Seta 

 Foot 



7/ 



Fig. 378. — Pellia epiphyllo. Longitudinal 

 section of sporogoniuni enclosed in 

 calyptra. 



The wall of the capsule when mature becomes two cells thick, whilst 

 from the archesporial tissue a large number of spore mother cells become 

 difl^erentiated. From each of these spore mother cells four spores are 

 produced. During this process the spore mother cells become deeply four- 

 lobed, with the lobes arranged pyramidally. The nucleus divides twice by 

 meiosis, and one of the four nuclei passes into each of the lobes, which 

 then separate and become difl^erentiated into spores. Meanwhile a certain 

 number of the archesporial cells, which have not become spore mother cells, 

 grow considerably in length and ultimately form long, thin, doubly-pointed 

 structures which are spirally thickened with ribs of cutin. These cells are 

 termed elaters. When young they probably assist in transferring food 

 material from the seta to the spores, but when the capsule is mature they 

 assist in the dissemination of the spores. 



It is important to realize that the whole of this development has been going 

 on within the venter of the archegonium, which has therefore enlarged 

 rapidly, keeping pace with the development of the sporogonium. For a 



