MOSSES. 



33^ 



ages are termed Teetb or Cilia, the whole together the Peristome; if the peristome is 

 wanting, the theca is said to be gymnostomous. The theca is at first a solid homo- 

 geneous mass of tissue ; the differentiation of its interior begins with the formation of 

 an annular intercellular space which separates off the wall of the theca consisting of 

 several layers of cells ; but the wall remains attached above and below to the colu- 

 mella. The intercellular space is traversed by rows of cells which stretch across from 

 the wall of the theca to the inner mass of tissue ; they resemble most nearly proto- 

 nemal filaments, or those of Algae, but have been formed by simple differentiation of the 

 tissue of the theca. They contain grains of chlorophyll like the inner cell-layers of 

 the wall. The outer layer of the wall of the theca is developed into a very character- 

 istic epidermis strongly cuticularised externally. The third or fourth layer of cells of the 

 inner mass of tissue, which is thus separated from the annular air-cavity by two or 

 three layers of cells (forming the spore-sac), produces the mother-cells oif the spores. 

 They are first of all distinguished by being densely filled with protoplasm, in which 



Pig. 24S.—Fu>iafia hyi^^roinetrica ; A a young leafy plant.;' 

 with the calyptra c; B a plant jC with the nearly ripe sporoj^o- 

 nium, s its seta, y the theca, c the calyptra ; C longitudinal sec- 

 tion of the theca bisecting it symmetrically ; rf operculum, 

 a annulus, / peristome, c c' columella, h air-cavity, s the pri- 

 mary mother-cells of the spores. 



Fig. 246. — Mouth of the theca of Fontinalis 

 (intipyretica ; afi outer peristome, 2 inner peri- 

 stome. After Schimper ( x 50). 



Hes a large central nucleus, and are attached without interstices to the surrounding 

 tissue in a parenchymatous manner. From their division proceed the spore-mother- 

 cells, which are isolated by the deliquescence of the cell-walls, and then swim in the 

 fluid contained in the spore-sac, till they form the spores by repeated division. The 

 Spore-sac is the term given to those layers of cells by which the large air-cavity is 

 separated from the spore-mother-cells. It seems convenient to consider the layers 

 which bound the spore-cavity on the axial side (Fig. 247, /) also as a part of the 

 spore-sac ; its cells contain on both sides starch-forming grains of chlorophyll. The 

 inner large-celled tissue, which contains but little chlorophyll, and is thus surrounded 

 on all sides by the spore-sac, is distinguished as the Columella. The spore-sac is ruptured 

 by the casting off of the operculum, but the columella remains dried up, and in Poly- 

 trichum there remains also a layer of cells, the Epiphragm, attached to the points of the 

 teeth of the peristome, and covering the opening of the theca. 



We must now examine somewhat more closely the origin of the Peristome. In 

 those genera w-hich, like Gymnostomum, do not form a peristome, the parenchyma 

 which fills up the inner space of the operculum is homogeneous and thin-walled ; when 



