GROUPS OF PLANTS. 49 



B, C). The fertile tissue in both the Liverworts and Mosses 

 is variously disposed; sometimes 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 arche- 

 sporium, or which in other cases separates the fertile tissue 

 into two parts, is known as the columella (Fig. 28, B, C). The 

 sporangium in the mosses is capsule-like and the spores are dis- 

 tributed in three ways : ( i ) In some cases the capsule does 

 n'ot 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. 28, 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 regulating the dispersal of the spores. In the sphag- 

 num 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. 28, E) vary in diameter from 10 to 20 

 microns, being sometimes larger. They occur in groups of four 

 in a mother-cell, and the spore-group is known as a tetrad, which 

 is characteristic for the Bryophytes and the higher groups of 

 plants. The spores therefore vary in shape from spherical tetra- 

 hedrons to more or less spherical bodies, depending upon the 

 degree of separation. The contents are rich in protoplasm and 

 oil (Fig. 28, F). The wall consists of two layers, the outer of 

 which is either yellowish or brown and is usually finely sculptured. 

 At the time of germination the outer wall is thrown off, and the 

 protonema develops (Fig. 28, G). The spores may germinate 

 almost immediately, or only after a considerable period. These 

 spores are asexual and each one is capable of giving rise to a 



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