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PLANT MORPHOLOGY 



where all the sporangia arise at the same time and therefore are all of the 

 same age, is found among the most primitive families, viz., Osmundaceae, 

 Schizaeaceae, and Gleicheniaceae. (2) The gradate sorus, in which the 

 sporangia arise in basipetal succession on an elongating receptacle, occurs 

 in the Hymenophyllaeeae, Dicksoniaceae, and Cyatheaceae. (3) The 

 mixed sorus, where sporangia of different ages are intermingled in the 

 same sorus and show no developmental sequence, is the most advanced 

 type. It is characteristic of all the Polypodiaceae except Woodsia and 



Onoclea, which are gradate. 



Sporangium. The sporangium in the Polypodiaceae 

 is slightly flattened and has a long, slender stalk (Fig. 

 241). It also has a rather long stalk in the Dicksoni- 

 aceae and Cyatheaceae, but in the lower families it has 

 a short, stout stalk or none. In all the Filicales the 

 sporangium wall consists of a single layer of cells. A 

 special feature is the presence of an annulus, a group 

 or, more commonly, a ring of specialized cells that 

 brings about the dehiscence of the sporangium. Its 

 cells have all but their outer walls thickened. Drying 

 causes a contraction of the thickened band, resulting in 

 a state of tension that finally ruptures the sporangium 

 wall. As the annulus bends backward, the spore mass 

 is exposed. Then, suddenly, the annulus springs to its 

 original position, hurling the spores into the air. 



The Osmundaceae have a rudimentary annulus (Fig. 

 242). In both this family and the Schizaeaceae the 

 annulus is apical, while in the Gleicheniaceae it is equatorial. Dehis- 

 cence in all three families is longitudinal. In the Hymenophyllaeeae, 

 Dicksoniaceae, and Cyatheaceae the annulus is oblique and the dehiscence 

 is obliquely lateral. In the Polypodiaceae the annulus is vertical, extend- 

 ing only about two-thirds of the way around the sporangium (Fig. 241). 

 Dehiscence is transverse. 



The Filicales resemble the Ophioglossales and Marattiales in being 

 homosporous, but differ in that they are leptosporangiate. This means 

 that the sporogenous tissue is developed from the outer segment arising 

 from the first periclinal division of the initial, rather than from the inner 

 segment, as among eusporangiate pteridophytes. The sporangium initial 

 consists of a single superficial or marginal cell that becomes papillate. A 

 periclinal division separates an inner cell from an outer cell, and then three 

 oblique walls appear in the latter in such a way that a tetrahedral apical 

 cell is formed. This cuts off a variable number of segments that form a 

 short stalk (Fig. 243A). The sporangium now enlarges above and, by 

 means of a periclinal division, an outer cap cell is separated from an inner 



Fig. 241. Mature 

 sporangium of one of 

 the Polypodiaceae, 

 showing the incom- 

 plete vertical annu- 

 lus, X150. 



