270 NATURE OF SPHAGNUM 



the subsequent introduction of other plants whose roots or stems 

 traverse the covering formed by the mosses. So frail is this 

 covering at first, that a very slight jar will cause the surface to 

 tremble and for this reason these places are known as quaking 

 bogs. The constant dropping of the decaying vegetation to the 

 bottom of the ponds, assisted by the drainage material, gradually 

 fills them, and in time they may become quite dry. The acid 

 character of these bogs prevents the entrance of those organisms 

 which promote decay. As a result, plants and animals that fall 

 into these bogs may only partly decay and remain for ages in a 

 wonderful state of preservation as is shown by the recovery of 

 skeletons of animals and clothing of men that belonged to a 

 prehistoric period. 



(a) The Structure and Reproduction of Sphagnum. — The 

 leaves of the bog moss are arranged in compact spirals around 

 the stems and consist of a single layer of cells as in the leafy 

 hepatics (Fig. 202, B). These cells, however, are of two kinds, 

 large and empty cells with spirally thickened walls which are 

 generally perforated with small pores and very narrow cells 

 containing chlorophyll. This distribution of the cells explains 

 the pale-green color characteristic of the sphagnums. Large 

 and spirally marked cells, similar to those of the leaf, also occur 

 in the cortex of the stem. The closely packed leaves enable the 

 bog mosses to take up water like a sponge and the sphagnums 

 are of considerable commercial value for this reason, being ex- 

 tensively employed by horticulturists to keep plants moist during 

 shipment. Perhaps these large cells may serve as floats, en- 

 abling the plant to bridge over ponds and they may enable the 

 plant to endure the acid waters in which these plants grow. In 

 this connection, it is noteworthy that rhizoids are entirely lack- 

 ing and that water and other substances are absorbed by the outer 

 cells of the stems and by certain branches which hang down in 

 wick-like strands close to the main stem (Fig. 201). 



The antheridia and archegonia are developed much as in the 

 leafy hepatics, the former organs appearing as stalked bodies 

 in the axils of the leaves on short cone-like branches and the 

 archegonia originate on the tips of short branches. The gameto- 



