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TEXTBOOK OF BOTANY 



Lea\ es usualh' mature in one season, though in some species they grow 

 for a period of three or more years. 



Fig. 308. Uncoiling of young leaves of cinnamon fern (Osmiinda cinnamomea) . 



The characteristic \'enation of ferns is dichotomous, but there are all 

 gradations from dichotom\' to a very complex net arrangement that is 

 better tenned reticulate. The lea\'es of a few species are somewhat suc- 

 culent and the venation is not easih' seen. Most of our common ferns 

 may be readilv recognized, howe\'er, b\^ their dichotomous venation and 

 the uncoiling of their voung leaves, known as circinate vernation. 



The fern leaf is similar in structure to that of the seed plant. The 

 epidemiis is a definite layer with stomates usualh' on the under side of 

 the leaf. Green plastids occur in the guard cells. The mesophyll is often 

 not differentiated into palisade and spongv tissues. The leaflets of some 

 of the film\' ferns are but a single cell la^er in thickness. Structurally, 

 the veins are quite like those of seed plants, and thev are branches of 

 the vein tissues of the stem and roots. 



The stems of most ferns are horizontal branched rhizomes. Thev mav 



