BRYOPHYTES. 



53 



that it will only take place from cells lying between 

 the peripheral cortex and the central conducting- 

 strand, as this region contains most reserve-stores 

 (PL III, figs. 3 and 4). The region in the seta 

 from which the protonemal threads arise corresponds 

 to the zone in the capsule situated between the 

 columella and the wall in which the spore-mother- cell 

 layer occurs. In the case of the vegetative stem it 

 is from an analogous region. The reason, he says, 

 why the peripheral cells of the seta do not form the 

 threads is because they become mature sooner and 

 lose their formative power. 



These phenomena observed by Stahl and Pring- 

 sheim constitute interesting examples of apospory. 

 But they do not appear to possess any significance 

 so far as affording support to the hypothesis of homo- 

 logous alternation of generations. As Celakovsky 

 points out, the vegetative sporogonial-tissues of the 

 moss have, with the exception of the peripheral layer 

 of cells, been sterilised, in the course of evolution, 

 from purely sporogenous tissue, so that it is not 

 surprising if many of the cells still possess the poten- 

 tiality of protonema-formation. This may be, in fact,, 

 regarded as a kind of reversion to an ancestral con- 

 dition, surely a rare statement to make with regard 

 to adventitious-growths in the vegetable kingdom. 

 The real reason why Pringsheim could not induce 

 protonema-formation from the peripheral cells is 

 because this layer has never been sporogenous, like 

 all the internal tissues have been. 



There may here be. introduced a phenomenon which 

 is of sufficient rarity to be regarded as an abnormality, 

 although it may in reality be normal for the particular 

 species in which it is found. 



Goebel has described, in the mosses Diphyscium, 

 Buxbaumia, and JEriopus remotifolius, the emission of 

 tubes from the surface-cells of the foot of the seta 

 which is embedded in the tissues of the gametophyte ; 

 the tubes become divided into cells by transverse walls, 



