182 MOSSES AND FERNS CHAP. 



and are at once distinguished from other Mosses by the dehis- 

 cence of their small capsules. These, like those of Sphagnum, 

 are raised upon a pseudopodium, and are destitute of a true 

 seta. The capsule opens by four vertical slits, which do not, 

 however, extend entirely to the summit (Fig. 94). This 

 peculiar form of dehiscence recalls the Jungermanniaceae, but is 

 probably only an accidental resemblance. The closely-set stems 

 branch freely; the leaves, with three-eighth divergence, are 

 either with a midrib (A. rupestris) or without one (A. 

 petrophila). 



The growth of the stem is from a pyramidal apical cell, as 

 in Sphagnum, and probably the origin of the branches is also 

 the same as in that genus. The growth of the young leaves is 

 usually from a two-sided apical cell, but another type of growth 

 is found where the apical cell is nearly semicircular in outline, 

 and segments are cut off from the base only. These two forms 

 of apical growth apparently alternate in some instances in the 

 same leaf. The originally thin walls of the leaf cells later be- 

 come thick and dark-coloured, whence the characteristic dark 

 colour of the plant. 



The stem in cross-section shows an almost uniform struc- 

 ture, and no trace of*the central conducting tissue of the higher 

 Mosses can be found. The outer cells are somewhat thicker- 

 walled and darker-coloured, but otherwise not different from 

 the central ones. Numerous rhizoids of a peculiar structure 

 grow from the basal part of the stem, and from these, new 

 branches arise, which replace the older ones as they die away. 

 These rhizoids are not simple rows of cells as in the Bryales, 

 but are either cylindrical masses of cells or flattened plates. 

 They penetrate into the crevices of the rocks, or apply them- 

 selves very closely to the surface, so that the plants adhere 

 tenaciously to the substratum (Ruhland (2)). 



Spores and Protonema 



The germination of the spores and the development of the 

 protonema show numerous peculiarities. The spores may 

 germinate within a week, or sometimes remain unchanged for 

 months. They have a thick dark-brown exospore and contain 

 chlorophyll and oil. The first divisions take place before the 

 exospore is ruptured, and may be in thrae planes, so that the 



