PRINCIPAL GROUPS OF PLANTS. 79 



archegonlum, which becomes somewhat extended until finally it 

 is ruptured. The dividing cells differentiate into a stalk and a 

 spore case or sporangium which is borne at the summit, the whole 

 structure being known as the sporogonium (Fig. 45). The 

 base of the stalk is embedded in the apex of the moss plant, 

 and is known as the foot, it being in the nature of a hausto- 

 rium or nourishing organ. As the sporogonium develops and 

 rises upward it carries with it the ruptured archegonium which 

 forms a kind of covering over the top, called the calyptra 

 (Fig. 45, ca). At first the sporangium is more or less uniform, 

 but eventually differentiates into two kinds of tissues, the one 

 being sterile and the other fertile (producing spores), which latter 

 is known as the archesporium (Fig. 45, B, C), The fertile tissue 

 in both the Liverworts and Mosses is variously disposed ; some- 

 times it forms a single area and is dome shaped, spherical, or 

 in the form of a half sphere. In other cases it is separated into 

 two areas by sterile tissue. The sterile tissue which extends up 

 into the dome-shaped archesporium, or which in other cases 

 separates the fertile tissue into two parts, is known as the 

 columella (Fig. 45, B, C). The sporangium in the mosses is 

 capsule-like and the spores are distributed in three ways: (i) 

 In some cases the capsule does not open, but when it decays the 

 spores are liberated. (2) In other cases the capsule dehisces 

 longitudinally in dry weather, and thus the spores are freed. 

 (3) There is a third method in which the capsule is provided 

 with a lid or operculum which comes off and permits the spores 

 to escape, this being the most common method for the escape 

 of the spores (Fig. 45, D). In the latter instance the mouth 

 of the capsule is usually marked by one or two series of cells, 

 constituting the peristome, which are teeth-like and characteristic 

 for some of the groups of mosses. These teeth bend inward or 

 outward, according to the degree of moisture, and assist in regu- 

 lating the dispersal of the spores. In the sphagnum mosses there 

 is no peristome, but, owing to unequal tension of the lid and capsule 

 on drying, the lid is thrown off, and the spores are sometimes 

 discharged with considerable force and sent to quite a distance 

 (as much as 10 centimeters), in this way insuring their dispersal. 

 The spores (Fig. 45, E) vary in diameter from 10 to 20 



