136 Practical Plant Biology. 



cap, the calyptra. As the sporangium ripens, underneath the 

 calyptra may be seen a tiny brown circular lid, the operculum. 

 Later the calyptra and then the operculum come away and allow 

 a fine brown dust to shake out from the sporangium. The grains 

 of this dust are the spores. 



When these spores are shed they are tetrahedral in shape and 

 each is covered with a brown outer wall. They absorb water and 

 become spherical. The brown outer layer breaks and the colour- 

 less inner layer of the wall is pushed out as a tubular cell. This 

 grows on and forms a filament of cells, in which there are usually 

 numbers of rounded chloroplasts embedded in the protoplasmic 

 film lying against the inner surface of the cell-wall. As the filament 

 grows it creeps along the surface of the soil and forms large numbers 

 of branches, some of which grow down into the soil. The cells of 

 the filament which are exposed to the light are rich in chloroplasts, 

 those which dip into the soil are colourless. The whole branch- 

 ing filament is called the protonema. If conditions are favourable 

 it will cover the soil with a rich green velvety mass. When the 

 protonema is well established, special small lateral pear-shaped 

 branches begin to be formed. These develop into the stem and 

 leaves of the moss. Microscopic study has shown the develop- 

 ment of the plant from these branches to be as follows : The end 

 cell of these branches is at first hemispherical. Three oblique 

 partitions arise in it, and form an apical cell of pyramidal shape, 

 the base of the pyramid being part of the hemispherical surface of 

 the original cell. As this cell enlarges segments are cut off from 

 each of its inner or convergent faces. The segments are then 

 divided by partitions parallel to the surface of the branch, into 

 inner and outer cells. The three inner cells divide repeatedly 

 and produce a column of thin-walled tissue, of which the axial 

 cells are long and narrow; and the outer are wider and somewhat 

 shorter. These form the inner tissues of the stem of the moss. 

 The outer cells formed from the first segments of the apical cell 

 give rise to the outer cells or cortex of the stem, and to the leaves. 

 The cortical cells develop thick brown walls and so the cortex 

 becomes a rigid tube encasing the stem and giving it stiffness. 

 As these tissues are being developed and differentiated from the 

 segments of the apical cell, that cell continues to grow and form 

 fresh segments so that growth is continuous. Leaves are formed 

 from certain of the outer segments of the apical cell. By repeated 

 divisions these segments form a small shelf of cells beside the 

 apex. This is a single layer of cells thick and one of the cells 

 centrally placed becomes an apical cell ; by subdivision this gives 



