36 



The Plant World. 



mass of mosses is constantly saturated with moisture, because 

 # the cells of the leaves are of two sorts, very large, clear, elliptical 

 cells with the walls spirally thickened and perforated by round 

 pores, and the true chlorophyll-bearing cells which are narrow 

 and elongated and lie between the others. The larger pore- 

 bearing empty cells, store large amounts of water, and the 

 spongy character of a handful of peat mosses is due to the 

 peculiar water-holding capacity of these cells. The upper 

 extremities of the moss stems continue their growth from vear 

 to year, while the lower portions die away, and if not completely 

 destroyed are converted into peat. Of the numerous lateral 

 branches arising from each of the shoots, some grow upwards 

 and form the apical tufts, while others turn downwards and 

 envelop the lower portions of the stem. Every year, by the 

 development of a stronger lateral branch, the stem becomes 

 forked; and by the death of the stem behind the fork, two new 

 plants may be formed eventually. The dead remains are 

 gradually compacted and peat is formed. In some cases, 

 peat is formed out of a pure growth of sphagnum, but in the 

 majority of cases, it consists of the remains of bog mosses 

 together with those of other plants which in part have been 

 enumerated above as bog plants. Even tree parts may be 

 incorporated in peat. Although the true peat bogs are confined 

 to the northern hemisphere, yet in the southern hemisphere, 

 according to Darwin and other observers, similar deposits are 

 formed in South America by Astelia pumila of the rush family, 

 and among the Chatham Islands, east of New Zealand, by 

 various flowering plants. The formation of peat depends upon 

 the slow and imperfect decay of the beg mosses and other plants 

 associated with the moss remains. This imperfect decomposi- 

 tion in turn is dependent upon the presence of so much moisture, 

 that the access of air is impeded, while the relatively low tem- 

 peratures in the bog also hinder the process of decay. The 

 compounds at first formed have antiseptic properties and these 

 soon put an end to further bacterial activity, and the process 

 of peat formation becomes a chemical one and extremely slow. 

 The solid decomposition products are brown substances, partly 

 soluble in water and imparting to it the color of strong tea and 

 an acid reaction. The acid reaction is due, says Hilgard,* to 



* Hilgard. E. W. Soils, page 122, 1906. 



