442 A TEXTBOOK OF THEORETICAL BOTANY 



Rotifers and other microscopic animals. They are the only perforated cells 

 on the branches. Although there is no definitely lignified vascular tissue in 

 Sphagnum, the plant contains lo to 13 per cent, of lignin in its dry weight. 



The power of water absorption by Sphagnum is remarkable and is an 

 important factor in its Ecology. The dry moss can absorb up to twenty-five 

 times its own weight of water. As it also transpires rapidly these reserves 

 of water are a valuable safeguard as well as affording a means of getting the 

 necessary nutriment from the very dilute solutions in which it lives. 



The leaves are inserted very closely together on the side branches, but 

 less so on the main stems. Their original arrangement is in three vertical 

 rows, corresponding to the three sides of the apical cell, from which the 



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Fig. 432. — Sphagnum sp. Part of a leaf showing 

 narrow chlorophyll-containing cells separated 

 by large empty cells with cuticularized ribs. 



segments are successively cut ofT. In the apical part of each stem this is 

 clearly seen, but torsions soon alter this arrangement into a spiral with 

 intervals of 144° between leaves. The base of each leaf springs from the 

 hadrome cylinder and passes through the spongy cortex. This leaf base 

 consists of cylindrical thick-walled cells, but the blade of the leaf, which 

 has no midrib, consists of two kinds of cells, both of which are much elongated. 

 The cells of one kind are wide and empty and resemble those of the cortex 

 in having open pores and strengthening bands of cuticle. The other kind 

 consists of very narrow living cells, with chloroplasts, which wind about 

 between the others, making a fine, green network (Fig. 432). Species of 

 Sphagnum are largely distinguished by minor variations of this structure, 

 but in its main outlines it is characteristic of the whole genus and is a unique 

 type of leaf (Fig. 433). 



The branches formed are of two kinds. Usually every fourth leaf on the 



