Lecture XVIII. 141 



mass within this layer forms the columella. The upper- end of 

 the columella is cone-shaped and projects into the slightly domed 

 operculum, while its lower end is much attenuated and joins the 

 axial tissue of the apophysis. 



Originally the tissue of the operculum is continuous with that 

 of the wall of the theca, but as differentiation proceeds a groove is 

 formed on the outside marking off the operculum and defining the 

 rim of the theca. Immediately above the rim a row of large cells 

 with easily broken radial walls is formed, and its outer walls 

 separate as a continuous band by the rupture of the radial walls. 

 This band is called the annulus. The separation of the annulus 

 leaves the operculum free to fall off. Like the wall of the theca 

 the operculum is a few cells thick. The lower surface of the 

 operculum is cut off from the top of the columella by a single layer 

 of cells, which is continuous with the inner layer of the wall of the 

 theca. The outer and inner walls of this layer of cells are strongly 

 thickened while the radial walls connecting these are thin and 

 early disappear, leaving the thickened walls as two separate mem- 

 branes over-arching the top of the columella. These two cone- 

 shaped membranes split with sixteen radial clefts from the apex to 

 the edge and so form two rows of sixteen teeth each attached to 

 the inside of the rim of the theca. This apparatus is called the 

 peristomium. 



As the theca matures, the cells of the sporogenous layer each 

 give rise to four spores; the thin-walled tissue of the columella 

 loses water and shrivels up, tearing open the space in which these 

 spores lie. It is at this moment the cell- walls of the annulus give 

 way and the operculum is cast off, exposing the peristomium. 

 The teeth of this latter are very sensitive to changes in atmospheric 

 moisture and by their hygroscopic curvatures alternately bend in 

 and out of the cavity of the theca, each time raking out a 

 small quantity of the ripe spores, thus scattering them in small 

 quantities over a considerable time. During this period the seta, 

 which is also hygroscopic and twisted, causes the theca to gyrate 

 irregularly. In this way the combined arrangements of the theca 

 form a very efficient mechanism for distributing the spores and 

 preventing them from falling in large masses which would lead to 

 waste by the overcrowding of the developing moss plants. The 

 dependence of the common species of'Funaria on the hygrometric 

 state of the atmosphere for the distribution of its spores is marked 

 by the specific title Funaria hygrometrica. 



With the production and shedding of the spores the develop- 

 ment of the whole apparatus consisting of the foot, seta and theca 



