DEVELOPMENT OF PLANTS 301 



the annulus, that lies in the groove just below the operculum, 

 As the walls of the capsule dry out at maturity they shrink, 

 compressing the air within the capsule until the tension ruptures 

 the delicate ring of cells and throws out the spores with an 

 audible report. No elaters are associated with the spores in any 

 of the mosses. 



The germination of the spores results in the formation of a 

 short chain or filament of cells which soon develop into a flat 

 and rather irregular thallus consisting of a single layer of cells 

 and attached to the ground by numerous rhizoids (Fig. 204). 

 This thallus may produce from its marginal cells other filaments 

 which behave like the original one and thus serve to increase the 

 number of thalli. The archegonia and antheridia are not de- 

 veloped upon these thalli, but there are first formed as in most 

 of the Jungermaniales, bud-like growths which develop into leafy 

 stems of the sphagnum (Fig. 204, b). These stems soon become 

 independent plants through the decay of the thallus and finally 

 bear the reproductive organs. 



109. Order b. Bryales. The Higher or True Mosses. The 

 great majority of plants, commonly known as mosses, belong 

 to this order. As a rule, these plants are of a higher type than 

 any of the other Bryophyta (Fig. 205). The stems are frequently 

 erect with radially arranged leaves and show considerable differ- 

 entiation of tissues, as is apparent in the cross-section of the stem 

 of one of the higher forms, shown in Fig. 206, A. Particularly 

 noticeable are the thin-walled cells located in the central strand 

 of the stem which serve to conduct the fluids and are therefore; 

 comparable to the tracheal tissue of the higher plants. A similar 

 conducting tissue is often seen in the central strand that appears 

 in the leaves of the majority of the Bryales (Fig. 206, B). In 

 connection with this higher development of the plant, attention 

 should be directed to the more efficient organs of absorption. 

 The rhizoids that spring from the base of the erect stems or from 

 the under side of forms with creeping stems are often larger than 

 those noted in the hepatics, and generally they are multicellular 

 and often twisted together into root-like strands (Fig. 206, C). 

 These features doubtless account in part for the larger size that 



